The Ampere and Electrical Standards

Abstract

This paper describes some of the major contributions to metrology and physics made by the NIST Electricity Division, which has existed since 1901. It was one of the six original divisions of the National Bureau of Standards. The Electricity Division provides dc and low-frequency calibrations for industrial, scientific, and research organizations, and conducts research on topics related to electrical metrology and fundamental constants. The early work of the Electricity Division staff included the development of precision standards, such as Rosa and Thomas standard resistors and the ac-dc thermal converter. Research contributions helped define the early international system of measurement units and bring about the transition to absolute units based on fundamental principles and physical and dimensional measurements. NIST research has helped to develop and refine electrical standards using the quantum Hall effect and the Josephson effect, which are both based on quantum physics. Four projects covering a number of voltage and impedance measurements are described in detail. Several other areas of current research at NIST are described, including the use of the Internet for international compatibility in metrology, determination of the fine-structure and Planck constants, and construction of the electronic kilogram.

Full text

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
[J. Res. Natl. Inst. Stand. Technol. 106, 65–103 (2001)]
The Ampere and Electrical Standards
Volume 106 Number 1 January–February 2001
Randolph E. Elmquist, Marvin E.
Cage, Yi-hua Tang, Anne-Marie
Jeffery, Joseph R. Kinard, Jr.,
Ronald F. Dziuba, Nile M.
Oldham, and Edwin R. Williams
National Institute of Standards and
Technology,
Gaithersburg, MD 20899-0001
randolph.elmquist@nist.gov
marvin.cage@nist.gov
yi-hua.tang@nist.gov
anne-marie.jeffery@nist.gov
joseph.kinard@nist.gov
ronald.dziuba@nist.gov
nile.oldham@nist.gov
edwin.williams@nist.gov
This paper describes some of the major
contributions to metrology and physics
made by the NIST Electricity Division,
which has existed since 1901. It was one
of the six original divisions of the National
Bureau of Standards. The Electricity Di-
vision provides dc and low-frequency cali-
brations for industrial, scientific, and re-
search organizations, and conducts research
on topics related to electrical metrology
and fundamental constants. The early work
of the Electricity Division staff included
the development of precision standards,
such as Rosa and Thomas standard resis-
tors and the ac-dc thermal converter. Re-
search contributions helped define the
early international system of measurement
units and bring about the transition to
absolute units based on fundamental princi-
ples and physical and dimensional mea-
surements. NIST research has helped to
develop and refine electrical standards
using the quantum Hall effect and the
Josephson effect, which are both based
on quantum physics. Four projects covering
a number of voltage and impedance mea-
surements are described in detail. Several
other areas of current research at NIST
are described, including the use of the In-
ternet for international compatibility in
metrology, determination of the fine-struc-
ture and Planck constants, and construc-
tion of the electronic kilogram.
Key words: calibration; electrical engi-
neering; Internet; josephson arrays; mea-
surement units; resistance measurements.
Accepted:
Available online: http://www.nist.gov/jres
Contents
1. Introduction .........................................................66
1.1 Origins of International Electrical Units ...............................67
1.2 Movement Towards Absolute Electrical Units...........................68
1.2.1 Development of Absolute Determinations .........................68
1.2.2 The Absolute Ohm ...........................................68
1.2.3 The Absolute Ampere ........................................70
1.3 Modern Electrical Standards at NIST .................................71
1.3.1 The Representation and Maintenance of the Ohm ..................71
1.3.2 The Representation and Maintenance of the Volt ...................71
1.3.3 Physical Constants ...........................................72
1.3.4 1990 Practical or Laboratory System of Units .....................72
65

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
1.4 Quantum Physics and Nobel Prizes...................................73
1.4.1 The Josephson Effect .........................................73
1.4.2 The Quantum Hall Effect......................................74
1.4.3 Ohm’s Law in Quantum Metrology .............................76
2. Some Present-day Electrical Measurement Programs at NIST .................77
2.1 DC Voltage ......................................................77
2.1.1 The NIST DC Voltage Standard Laboratory.......................77
2.1.2 Industrial Needs in Voltage Metrology ...........................78
2.1.3 Future Developments in Voltage Metrology .......................80
2.2 AC-DC Thermal Transfer Instruments ................................80
2.2.1 Development of the Thermal Converter at NIST ...................80
2.2.2 Thin-Film Multijunction Thermal Converters ......................81
2.2.3 Cryogenic Thermal Converter ..................................81
2.3 Impedance Measurements ..........................................82
2.3.1 Development of the Calculable Capacitor at NIST .................82
2.3.2 The NIST Calculable Capacitor.................................84
2.3.3 Realization of the SI Ohm: the Calculable Capacitor Chain ..........86
2.3.4 Maintenance of the Capacitance Unit from the Calculable Capacitor ...87
2.3.5 Impedance Calibration Laboratory...............................87
2.3.6 Extension of Measurement Frequency............................87
2.3.7 AC QHR Measurements.......................................88
2.4 DC Resistance....................................................89
2.4.1 Quantized Hall Resistance Measurements.........................89
2.4.2 Standard Resistors ...........................................89
2.4.3 Techniques Used for Resistance Calibrations ......................91
2.4.4 Resistance Scaling ...........................................93
3. Electrical Metrology at NIST in the Twenty-first Century ....................94
3.1 Telemetrology....................................................94
3.1.1 SIMnet.....................................................94
3.1.2 Internet-Assisted Measurement Assurance Program .................96
3.2 The Absolute Ampere and the Quantum Age...........................96
3.2.1 The Moving Coil Watt Balance.................................96
3.2.2 A Description of the 1990s NIST Watt Experiment.................98
3.2.3 New Construction and Monitoring of the Kilogram.................99
4. Conclusion .........................................................100
5. References .........................................................100
1. Introduction
As a prelude to this article, we note that about 100
years before the National Bureau of Standards was cre-
ated in 1901, Alessandro Volta found that an
“electromotive force”(emf) was produced in an electri-
cal circuit containing dissimilar metals and an elec-
trolyte solution. This discovery allowed Volta to con-
struct the world’s first electric battery. Experiments
with electric current from a “Voltaic pile”led Georg
Simon Ohm to discover the empirical relation known as
Ohm’s law. In modern terminology [1] the equation that
Ohm first published in 1826 is
I=
␥
A
lV.
The factor
␥
is now called the conductivity of the mate-
rial, with Vthe electromotive force across a conductor
of length land constant cross-sectional area Awhen a
current Iflows. Ohm’s law can be written in terms of the
electrical resistance Rof a particular conductor as
V=IR.
In The´orie mathe´matique des phe´nome`nes e´lectro-dy-
namiques (1872), Andre Marie Ampe`re made deduc-
tions from experiments on magnetic fields and electric
currents. Ampe`re proved that two currents produce a
mutual attraction or repulsion. His formula (in present
day form) for the force between two current-elements
contains a constant of proportionality
␮
0, which is the
magnetic constant (also called the permeability of
66

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
vacuum). The value now assigned to this constant
␮
0≡4␲⫻10⫺7N/A2defines the ampere in such a way
that electrical and mechanical measurements of quanti-
ties such as power or energy are equivalent.
1.1 Origins of International Electrical Units
Electrical quantities are defined by principles of elec-
tromagnetism discovered by Ampe`re, Gauss, Faraday,
Weber, and other scientists in the first half of the 19th
century. Early in the study of electrical phenomena,
scientists attempted to systematize these quantities and
relate them to common measurements. To compile an
orderly system of such units, the British Association for
the Advancement of Science established the Committee
on Electrical Units and Standards in 1861. A framework
for the modern electrical units was set up under the
guidance of the chairman, J. Clerk Maxwell. Maxwell
recognized that in both electrical and mechanical units
certain quantities appear, thus these common physical
units (power and energy) should represent the same unit
magnitude in either system. In 1863 the committee rec-
ommended a set of “absolute practical”electrical units
[2, 3] and defined their magnitudes. The set of electrical
units that were selected was as follows:
ampere = unit of current,
volt = unit of emf,
joule = unit of energy,
coulomb = unit of charge,
ohm = unit of resistance,
watt = unit of power.
Metrologists then developed experimental means to
standardize their electrical measurements, and an
“international-reproducible”system was adopted using
the best scientific measurements of the time. With re-
spect to the ohm, the Chicago Electrical Congress of
1893 chose to recognize the resistance of a column of
mercury 1.063 m long and of constant 1 mm2cross
section, at a temperature of 0 ⬚C as the agreed-upon 1 ⍀
standard. This was slightly modified for use in the
United States in 1894, with the mass of the column
defined as 14.4521 g, instead of defining the constant
cross-sectional area [4]. The international ampere
adopted at a meeting of the British Association in Edin-
burgh (1892) was the current that would deposit silver
from a silver nitrate solution at a rate of 0.001118 g/s
under specified conditions, using a silver “voltameter”
[2]. These two experiments were formally recognized
until the 1930s as the basis of the international ohm and
ampere. Since a volt is the difference of potential pro-
duced across an ohm by a current of one ampere, only
two of the three units required an experimental defini-
tion.
In 1875, the Convention of the Meter (Treaty of the
Meter), a diplomatic agreement to harmonize measure-
ments and measurement standards internationally, was
signed in Paris by representatives of seventeen of the
world’s most industrialized nations. The Convention es-
tablished the General Conference on Weights and Mea-
sures (CGPM, a diplomatic organization), an interna-
tional metrology laboratory, the International Bureau of
Weights and Measures (BIPM, Bureau International des
Poids et Mesures), and the International Committee for
Weights and Measures (CIPM). This formal structure
continues to this day as the mechanism for establishing
and making changes in international measurement stan-
dards. Initially the CIPM/BIPM concentrated on mass
and dimensional measurements and international elec-
trical metrology proceeded independently, as national
measurement institutes (NMIs) were created.
Following the signing of the Meter Convention, indus-
trialized nations discovered that a need existed for na-
tional measurement laboratories, where standards could
be maintained and compared, and physical constants
could be determined. National standards laboratories
were organized by Germany, France, and Great Britain,
followed closely by the United States, in order to pre-
serve and disseminate the units of measure. The Na-
tional Bureau of Standards (renamed the National Insti-
tute of Standards and Technology, or NIST, in 1988) was
created in Washington, DC, in 1901, with Division II to
be devoted to electricity [4].
At the 1908 International Conference on Electrical
Units and Standards in London, it was recommended
that representatives of the NMIs should meet and agree
on new values of international units as defined by the
mercury ohm and silver voltameter. In April and May
1910, the International Technical Committee met in
Washington, DC at NBS. Scientists from Germany,
France, and Great Britain brought standard resistors and
standard cells that had been carefully evaluated in terms
of their national units. Dr. E. B. Rosa was the first Chief
of the Electricity Division and headed this committee.
Comparisons made at the meeting showed that the resis-
tance unit represented by the German standards was
only 1 ⫻10⫺5larger than that of the British. Results of
work then in progress under Dr. Rosa were in reasonable
agreement, and the committee recommended that all
countries use, as the international ohm, the mean of the
values found by Germany and Great Britain [2, 5]. Us-
ing the average emf obtained with silver voltameter ex-
periments from the four nations, the Committee also
recommended that the voltage of the Weston normal cell
(described in Sec. 1.3.2) at 20 ⬚C be assigned a value
of 1.0183 international volts. The precision of the mea-
surements was somewhat higher, however, with the av-
erage international value being 1.018312 (8) V [5]. On
67

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
January 1, 1911, the system of “international units”was
inaugurated, based on the Committee results. As a
byproduct of the work of the International Technical
Committee of 1910, NBS came into possession of a
group of 36 standard cells, part of the large group of
Weston cells that had been used to assign this value to
the Weston Normal Cell [2].
1.2 Movement Towards Absolute Electrical Units
In 1921, the Sixth CGPM modified the Meter Con-
vention to extend the scope of the BIPM and CIPM to
cover electrical measurements. This decision was imple-
mented in 1927 with the formation of the Consultative
Committee on Electricity (CCE), an advisory body of
international experts, which formulated the responsibil-
ities of the BIPM in the area of electricity. The program
went into effect in 1929.
The CIPM decided in 1929 that it was no longer
possible to maintain the international units with the de-
gree of accuracy demanded by science using the existing
“international”definitions. The national laboratories
were finding it more and more difficult to maintain the
international units based on independent measurements
of the mercury ohm and silver ampere. Problems as
fundamental as the existence of silver isotopes were
known to cause errors. In addition, the corrections that
had to be applied for the difference between interna-
tional and absolute units were becoming appreciable in
calorimetry, thermometry, and measurements of funda-
mental constants [2]. At the Eighth CGPM in 1933, the
consensus among representatives was that new types of
“absolute ohm”and “absolute ampere”experiments
should be used to define the three main electrical units.
These experiments would be based on the interactions
between mechanical and electrical phenomena. As
Maxwell had pointed out, this would make electromag-
netic quantities—such as the ampere, volt, and ohm—
fully consistent with other units in science and engineer-
ing. There were already in existence several promising
methods for relating these quantities to the mechanical
units of length, time, and mass. Since the experiments
required careful electrical and dimensional measure-
ments as well as complex calculations, the new
“absolute determinations”would be pursued primarily
at the large NMIs.
1.2.1 Development of Absolute Determinations
H. L. Curtis and R. W. Cur tis published “An Absolute
Determination of the Ampere”in the Bureau of Stan-
dards Journal of Research in 1934 [6]. This measure-
ment used a Rayleigh current balance, in which the
electromagnetic force between concentric coils is bal-
anced by the gravitational force on a mass. Within 4
years, two absolute-ohm determinations were com-
pleted by H. L. Curtis, C. Moon, and C. M. Sparks [7,
8]. They computed the value of inductors in absolute
units from dimensional measurements and from the per-
meability of the surrounding medium, and measured
those values in international units, based on fixed stan-
dards of resistance.
In the 1920s, Dr. James L. Thomas had taken up the
task of improving the long-term stability of wire-wound
resistors, which were used to measure the current in
absolute determinations. When a resistor is made by
winding wire on a spool, parts of the crystalline struc-
ture of the wire are stressed past their elastic limit.
Thomas developed wire-wound standard resistors that
were annealed at high temperature, which released some
of the internal strains and reduced the rate of change of
resistance with time [1]. Heat-treated manganin wire
resistors developed by Thomas incorporated hermeti-
cally-sealed, double-walled enclosures, with the resis-
tance element in thermal contact with the inner wall of
the container to improve heat dissipation. These 1 ⍀
Thomas-type standards [9, 10] proved to be quite stable
with time, and quickly came into favor as the primary
reference for maintaining the resistance unit at NBS and
at many other NMIs (see Fig. 1).
By 1944, three absolute-ampere and three absolute-
ohm experiments were completed at the Bureau [11],
and similarly accurate absolute determinations of the
ampere and ohm were available from Britain. At that
point (see Fig. 2), according to H. L. Curtis [11],
“during the last two decades not one of the national
laboratories has reestablished its unit for the interna-
tional ohm by making measurements upon the mercury
ohm. Instead each laboratory maintains its international
ohm by means of wire resistance standards.”Thus, after
World War II, the CIPM approved long-awaited new
recommendations for the electrical units. These values
were based on the absolute determinations, and super-
ceded the mercury ohm and silver ampere on January 1,
1948 [2]:
1 mean international ohm = 1.000 49 (pre-1948)
absolute ohm;
1 mean international volt = 1.000 34 (pre-1948) absolute
volt.
1.2.2 The Absolute Ohm
Work continued on improving the absolute measure-
ments of electrical units and, in 1949, J. L. Thomas, C.
L. Peterson, I. L. Cooter, and F. R. Kotter published a
new measurement of the absolute ohm [12] using an
inductor housed in a non-magnetic environment (see
68

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 1. A double-walled 1 ⍀standard resistor of the Thomas type.
Fig. 2. Differences from the adjusted mean values of the International Ohm, as maintained by various national
laboratories, 1911 to 1948. Points marked “Hg”indicate the results of measurements on mercury columns. Reprinted
from Ref. [2].
69

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 3). Using the Wenner method of measuring a resis-
tance in terms of a mutual inductance and a rate of
rotation, their work gave a value of 0.999 994 absolute
ohm for the new as-maintained unit of resistance at
NBS. The mean value assigned to ten Thomas-type
standard resistors from this experiment was found to
have been the same between 1938 and 1948 to within
1␮⍀/⍀. Thomas et al. wrote in a 1949 paper, this was
“the first satisfactory method that has been devised for
checking the stability of the unit as maintained by a
group of wire-wound resistors.”
In 1956, Thompson and Lampard at the Australian
national metrology laboratory determined that the value
of a special type of cross-capacitor was dependent only
on the length of the capacitor, the speed of light, and the
permeability of free space. Based on this principle, the
calculable cross-capacitor could provide an alternative
method of evaluating the unit of resistance based on
straightforward measurements of length and time. R. D.
Cutkosky developed this absolute capacitance standard
and the complex impedance-bridge techniques needed
to transfer the results to the ohm unit maintained at NBS
[13]. Cutkosky obtained the value of the U.S. Legal
Ohm in absolute units with a relative standard uncer-
tainty of 2.1 ␮⍀/⍀in 1961, using gauge bars as elec-
trodes for the capacitor. This measurement [13, 14]
indicated that the value of the NBS unit of resistance
was 1.000 000 6 ⍀on that date. A more precise cross-
capacitor constructed in the late 1960s has yielded a
number of improved values of the farad, the ohm, and of
related fundamental constants. In 1974, Cutkosky’s cal-
culable capacitor chain (see Sec. 2.3.3) yielded a value
of the ohm with a relative standard uncertainty of
0.03 ␮⍀/⍀.
1.2.3 The Absolute Ampere
Improved absolute measurements of current were in
some ways more difficult than those of the ohm, and
proceeded by smaller steps. Before WW II, at about the
same time that the moving-coil current balance was
being used to determine the ohm, H. L. Curtis and R. W.
Curtis had started to prepare a balance of a special
design for the absolute ampere determination. In 1958
R. L. Driscoll reported results from this Pellat balance
[15]. The mechanical measurement was of the torque on
a small coil, with axis at right angle to the magnetic field
of a large horizontal solenoid. When the current passing
through the small coil was reversed, it produced a force
that could be balanced by a mass of about 1.48 g placed
on the balance arm. The large stationary coil was wound
on a fused-silica former and the balance beam was
Fig. 3. A photograph of the outdoor pier with a wooden model showing the location of the mutual inductor, from the 1949 report “An absolute
measurement of resistance by the Wenner method”[16].
70

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
equipped with knife-edges and supports machined from
natural agate. The effect of the measured dimensions of
the small coil on the computed mutual inductance was
the largest contribution to the uncertainty, which totaled
about 8 ␮A/A. Also contributing to the uncertainty
were the determination of the balancing mass and of the
acceleration due to gravity.
As soon as possible after completing the Pellat bal-
ance measurement, Driscoll and Cutkosky [16] repeated
the 1934 Rayleigh current-balance determination of the
ampere using the original apparatus. The results of
these two experiments, 1 NBS ampere = 1.000 013
⫾0.000 008 absolute amperes by the Pellat method,
and, 1 NBS ampere = 1.000 008 ⫾0.000 006 absolute
amperes by the current balance, were in good agree-
ment. This gave an overall relative uncertainty in the
ampere at NBS of about 5 ␮A/A at that time (1958), and
verified that the ratio of emf over resistance of the main-
tained standards had been constant to within about one
partin10
5since 1942.
1.3 Modern Electrical Standards at NIST
1.3.1 The Representation and Maintenance of the
Ohm
The value of the U.S. representation of the ohm or
“legal”ohm maintained at NIST has been adjusted only
twice. This occurred once in 1948 when the ohm was
reassigned using a conversion factor relating the interna-
tional reproducible system of units [3] to the precursor
of the International System of Units (SI) derived from
the fundamental units of length, mass, and time. The
second reassignment was in 1990 when the ohm became
based on the quantum Hall effect (QHE) described in
Sec. 1.4.2. After 1960, ohm determinations were made
using calculable capacitors based on the Thompson-
Lampard theorem and a sequence of ac and dc bridges
(see Sec. 2.3). Then came the discovery of the QHE in
1980, which has provided an invariable standard of re-
sistance based on fundamental constants. Consequently,
on January 1, 1990 the U.S. Legal Ohm was re-defined
in terms of the QHE, with the internationally-accepted
value of the quantum Hall resistance (or von Klitzing
constant, after the effect’s discoverer) based on calcula-
ble capacitor experiments and other fundamental con-
stant determinations. At that time, the value of the U.S.
Legal Ohm was increased by the fractional amount
1.69 ⫻10⫺6to be consistent with the conventional value
of the von Klitzing constant [17].
From 1901 to 1990, the U.S. Legal Ohm was main-
tained at 1 ⍀by selected groups of manganin resistance
standards [18]. Four different types of resistance stan-
dards have been represented in these groups, whose
numbers have varied from 5 to 17 resistors. From 1901
to 1909, the group comprised Reichsanstalt-type [19]
resistance standards made by the Otto Wolff firm in
Berlin. These standards were not hermetically sealed
and consequently underwent changes in resistance as a
function of atmospheric humidity. In 1907 Rosa cured
the problem by developing a standard whose resistance
element is sealed in a can filled with mineral oil [20].
The U.S. representation of the ohm was maintained by
ten Rosa-type 1 ⍀resistance standards from 1909 to
1930. Over the years, measurements of differences be-
tween the individual Rosa-type resistors indicated that
the group mean was probably not constant. Thomas in
1930 reported on the development of his new design for
a resistance standard having improved stability [9]. The
Thomas resistance standards were more stable immedi-
ately following construction than the Rosa-type resistors
and two were added to the primary group in 1930.
Eventually, in 1932, the Rosa-type resistors in the pri-
mary group were replaced by the Thomas resistors. To
reduce loading errors, Thomas in 1933 improved the
design of his resistor by using manganin wire of larger
diameter mounted on a larger diameter cylinder to in-
crease the dissipation surface area [10]. A select group
of the new-design Thomas resistors was used to main-
tain the U.S. Legal Ohm from 1939 until its re-defini-
tion in 1990 based on the QHE.
1.3.2 The Representation and Maintenance of the
Vo l t
For almost 80 years starting in 1901, the U.S. Legal
Volt was maintained by several groups of standard cells.
There was a large effort in the late nineteenth century
and the early twentieth century to establish a standard for
electromotive force (emf) based on electrochemical re-
actions within chemical cells. The first legal unit of
voltage for the United States was based on the Clark
cell, developed by Latimer Clark in 1872 [21], with its
output assigned a value of 1.434 international volts by
the 1893 International Electric Congress. Public Law
105, passed by the U.S. Congress in 1894, made this the
legal standard of voltage in the U.S. During the years
between 1893 and 1905, the standard cell devised by
Edward Weston was found to have many advantages over
the Clark cell [2]. The Weston cell consists of a cad-
mium amalgam anode and a mercury-mercurous sulfate
cathode with a saturated cadmium sulfate solution as the
electrolyte. In 1908 at the London International Confer-
ence on Electrical Units and Standards, the Weston cell
was officially adopted for maintaining the volt. After
1908 only Weston cells were used for maintaining the
national standard in the United States.
71

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
The Weston standard cell can be disturbed by trans-
port or if subjected to a change in temperature or a small
electrical current. When at times it was necessary to
eliminate cells—due to changes in emf of a cell relative
to the mean of the group—new cells could be added. In
1965 the National Reference Group of standard cells
[22] included eleven cells made in 1906, seven cells
made in 1932, and 26 cells made in 1948. Long-term
stability of the volt reference was also maintained by
comparisons of neutral and acid cells, preparing and
characterizing new cells, and through international com-
parisons and absolute ampere and ohm experiments.
According to Driscoll and Olsen [23], the results of the
absolute current balance measurements could be re-
garded “as assigning a value to the emf of the standard
cell used to control the strength of the current”andasa
check on the emf of the NIST standard cell bank. The
use of the Weston cell as the national standard of voltage
was supported by a considerable amount of research in
electrochemistry and related fields at NBS as seen by
the staffing below.
Performance of standard cells R. J. Brodd, W. G. Eicke. H. E.
Ellis, C. E. Waters, F. A. Wolfe
Chemistry and thermodynamics L. H . Brickwedde, D. N. Craig,
W. J. Hamer, P. E. Robb, G. W.
Vinal, F. E. Vinal
Experimental cells V. E. Bower, W. J. Hamer, C. A.
Law, G. Roberts, A. S. Skapars,
G. W. Vinal
Instrumentation H. B. Brooks, W. G. Eicke, B. F.
Field, F. K. Harris, P. A. Lowrie,
B. A. Wyckoff
Work in this area through 1964 is summarized in NBS
Monograph 84 [22].
Before the Josephson effect was discovered (see Sec.
1.4.1), it was difficult to provide incontrovertible evi-
dence regarding the long-term stability of the U.S. Le-
gal Volt. However, considerable evidence indicated that
the unit of emf preserved with standard cells was un-
likely to have changed by any significant amount, rela-
tive to the best measurements of the time, from the early
1900s to the 1960s.
In the late 1950s, research in solid-state physics stim-
ulated the growth of the semiconductor industry. A new
type of voltage standard based on a solid-state device,
the Zener diode, appeared in the early 1960s. W. G.
Eicke at NBS first reported the possibility of using
Zener diodes as transport standards [24]. In the follow-
ing years, after several manufacturers started making
commercial Zener voltage standards, these references
began to replace standard cells in commercial use. Al-
though Zener voltage standards exhibit higher noise
characteristics than standard cells, and are affected by
environmental conditions of temperature, atmospheric
pressure, and relative humidity, they are now widely
used in many metrology labs because of their robust
transportability.
1.3.3 Physical Constants
In the early 1950s, director E. U. Condon supported
a greater emphasis on physics and basic standards at
NBS. The Electricity and Optics Division began prepa-
rations for measuring the fundamental constant
␥
p, the
gyromagnetic ratio of the proton, which would relate the
measurement of magnetic field strength to the nuclear
magnetic resonance (NMR) frequency of the proton.
This measurement grew out of the work on the measure-
ment of relative positions of currents in precision so-
lenoids used in the ampere determinations. Driscoll and
Olsen noted [23], “it was kept in mind during the con-
struction that (the Pellat) solenoid might be used to
provide a magnetic field for a later measurement of
␥
p.”
The single-layer solenoid together with a set of
Helmholtz coils did provide a uniform field region suit-
able for the NMR measurements used to measure
␥
p
(denoted by
␥
'pin water samples). Early measurements
were made by the method of free precession, and later
ones were done by the nuclear induction method. These
measurements yielded a final result in 1979 [25], with a
relative standard uncertainty of about 2.1 ⫻10⫺7.
An entirely new apparatus was begun thereafter to
measure
␥
'pin low magnetic field, and to help test the
theory of the Josephson effect, quantum electrodynam-
ics, and the newly discovered quantum Hall effect. The
improvements in this apparatus included a method of
injecting compensation currents into selected solenoid
windings, which reduced the need for measurements of
the mean solenoid diameter [26]. The dimensional mea-
surements, the NMR measurements, and the various cal-
ibrations contributed about 0.5 ⫻10⫺7each in relative
uncertainty. The result, for protons in a spherical sample
of water at 25 ⬚C, was
␥
'p(low) = 2.67 513 376 ⫻108s⫺1T⫺1NIST,
with a total relative uncertainty of 1.1 ⫻10⫺7. The flux
density was measured in terms of the NIST laboratory
units of resistance and voltage at the time.
1.3.4 1990 Practical or Laboratory System of Units
NIST determinations of physical constants, namely
␥
'p[26], the Josephson frequency-to-voltage ratio 2e/h
[27], the quantized Hall resistance (QHR) in terms of a
resistance derived from the calculable capacitor [28,
29], and the moving-coil force-balance determination of
the watt [30], were analytically combined [31] in 1989
72

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
to obtain values of certain fundamental constants. These
results made a significant contribution to the 1990 rec-
ommended values of the Josephson and von Klitzing
constants that were used to establish laboratory repre-
sentations of the volt and ohm (based on the Josephson
and quantum Hall effects) that were consistent with the
SI. By international agreement, Josephson and QHE
devices are now the preferred reference standards for
measurements of voltage and resistance. A third quan-
tum physics effect, based on “single electron tunneling”
(SET), could lead to an absolute electron-counting
device based on quantum physics and further improve-
ments for the units farad and ampere. Contributions to
the physical and metrological understanding of these
quantum-effect devices are discussed in the following
section.
1.4 Quantum Physics and Nobel Prizes
Two Nobel Prize-winning discoveries in condensed
matter physics, the Josephson effect [32] by Brian D.
Josephson in 1962 and the QHE [33] by Klaus von
Klitzing in 1980, have provided significant improve-
ments in how the SI volt and ohm are maintained and
disseminated. This section describes the early develop-
ment of these two effects into intrinsic voltage and resis-
tance standards whose values, under proper operating
conditions, depend only on fundamental constants of
nature.
1.4.1 The Josephson Effect
In 1962, Brian Josephson, a graduate student at Trin-
ity College, Cambridge, England, predicted [32] that
electrons can tunnel in pairs (Cooper pairs) between two
superconductors separated by a thin insulating barrier (a
weak link or Josephson junction). An applied dc voltage
Vacross the barrier would generate an ac current at the
frequency f=2eV /h, where eis the elementary charge
and his the Planck constant. Conversely, an applied ac
current of frequency fwould generate a dc voltage Vn
(see Fig. 4) at the quantized values
Vn=nh f/2e, (1)
where nis an integer and the value of 2e/his approxi-
mately 483.6 MHz/␮V. Josephson wrote his Ph.D. the-
sis on this theory [34], which won a share of the 1973
Nobel Prize in physics.
Anderson and Rowell [35] provided the first experi-
mental verification of the theory by observing a dc
current (a supercurrent) across a tunnel barrier with no
applied dc voltage. Shapiro [36] then obtained the con-
stant voltage steps Vnof Eq. (1) by using rf microwaves
Fig. 4. Josephson junction structure irradiated by microwaves of en-
ergy hf, producing a dc voltage Vnacross the junction.
to generate an ac current of frequency facross the super-
conductors (see Fig. 5 for an example). Clark [37]
showed in 1968 that the value of 2e/hobtained from
Josephson effect measurements was material-indepen-
dent to within 1 part in 108by applying the same mi-
crowave radiation to pairs of dissimilar Josephson junc-
tions and comparing the junction voltages. In 1968,
Parker, Langenberg, Denenstein, and Taylor [38] com-
pared, via a potentiometer, the Josephson voltages of
junctions consisting of five different superconducting
materials and various combinations of thin-film tunnel
junctions or point contacts with 1.018 V Weston satu-
rated standard cells [22] calibrated by NBS. They ob-
tained a value of 2e/hwith a one-standard-deviation
fractional uncertainty of 3.6 ⫻10⫺6. Finnegan et al.
[39] reduced this uncertainty to 1.2 ⫻10⫺7in 1971.
Fig. 5. A series of steps of constant voltage generated by a Josephson
junction array.
73

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
It was argued on fundamental grounds by Bloch [40]
and Fulton [41] that Eq. (1) must be exact. The use of
superconducting-quantum-interference device (SQUID)
null detectors in the early 1970s allowed this to be tested
to a few parts in 109[42, 43] and thus the Josephson
effect had obvious potential for use as a voltage standard
[44]. By the early 1970s, NIST staff had set up a poten-
tiometric measurement system in Gaithersburg that
compared 2 mV to 10 mV dc Josephson junction
voltages with 1.018 V standard cells to within a few
times 10⫺8[45, 46]. International comparisons in 1971-
72 between NMIs including NBS, the BIPM, the Na-
tional Physical Laboratory (NPL) in the U.K., the Na-
tional Research Council (NRC) in Canada, the National
Measurement Laboratory (NML) in Australia, and the
Physikalisch-Technische Bundesanstalt (PTB) in Ger-
many found that the measured values of 2e/hagreed
with each other to within 2 ⫻10⫺7[47].
These results from the NMIs suggested the course of
adopting a value of 2e/hfor use in maintaining units of
voltage. The U.S. was the first nation to do this, and the
value of 2e/hto be used at NBS was chosen to prevent
a discontinuity when NBS conver ted from standard cells
to the Josephson effect [48]. NBS began maintaining
and disseminating the U.S. volt based on the Josephson
effect in July, 1972 using a 10 mV measurement system
with an uncertainty of 2 ⫻10⫺8[46]. Soon after, the
Consultative Committee on Electricity of the CIPM rec-
ommended the value KJ⫺72 = 483 594 GHz/V, which all
countries adopted except the United States, France, and
the Soviet Union.
In many applications, Josephson junctions were un-
doubtedly better references than standard cells, which
are sensitive to environmental conditions, can shift val-
ues on transport, and can drift by a few times 10⫺8per
year. The typical 5 mV to 10 mV reference output from
early Josephson devices made from a few junctions re-
quired both very low-level voltage balances and scaling
by a factor of 100, both of which seriously limited the
accuracy of measuring 1.018 V standard cells.
Then in 1977, Levinson [49] showed that unbiased
Josephson junctions would spontaneously develop quan-
tized dc voltages when irradiated with microwaves,
opening the path to successful Josephson junction ar-
rays. C. A. Hamilton, R. L. Kautz, F. L. Lloyd, and
others of the NBS Electromagnetic Technology Division
at Boulder began developing and improving Josephson
standards based on series arrays of junctions operated
near zero dc voltage bias [50, 51]. Elsewhere, Tsai et al.
[52] found in 1983 that the constant of proportionality
between the voltage and frequency is the same to at least
2⫻10⫺16 for two different kinds of Josephson junc-
tions.
Stable 1 V zero-crossing arrays were operating at
NBS [53] and PTB [54] by 1985, using about 1500
junctions and rf fields of 70 GHz to 90 GHz. Arrays with
output voltages at the level of 1 V soon were used in
NMIs throughout the world [55, 27]. By 1989 NIST had
made a 19 000 junction, 12 V array [56]. The wide-
spread use of Josephson junction arrays in national stan-
dards laboratories, and better SI determinations of 2e/h,
led the CCE to recommend [57] a new exact conven-
tional value for the Josephson constant:
KJ⫺90 = 483 597.9 GHz/V, (2)
which is fractionally larger by 8 ⫻10⫺6than the 1972
conventional value. The new value was adopted world-
wide on January 1, 1990, and thereby became the new
basis for the U.S. Legal Volt. This definition of KJ⫺90 is
the present volt representation, based on an ideal
Josephson voltage standard. The conventional value was
assumed by the CCE to have a relative standard uncer-
tainty of 0.4 ␮V/V. By convention, this uncertainty is
not included in the uncertainties of the representation of
the volt, since any offset from the SI volt will be consis-
tent between different laboratories using the Josephson
effect standard.
1.4.2 The Quantum Hall Effect
In the classical Hall effect, a current of particles with
charge qand velocity vis passed through a device
placed in a magnetic field with perpendicular magnetic
flux density component B. A Lorentz force qvB deflects
the conducting charges toward one side of the device.
The resulting charge redistribution produces an electric
field Eacross the device. At equilibrium, a Coulomb
repulsion force qE opposes the Lorentz force. The elec-
tric field generates a Hall voltage proportional to B,
perpendicular to both the magnetic flux and the flow of
current.
The QHE is seen only if the conducting particles
(usually electrons) are confined to a two-dimensional
sheet within the device by a potential that restricts their
out-of-plane motion. In the integer QHE they can be
treated as independent Fermi particles, and thus at low
temperature form a two-dimensional electron gas
(2DEG). This 2DEG is indicated by the light-blue region
of the semiconductor device shown in the inset of Fig.
6. Once again, the Lorentz force resulting from the
applied magnetic field equals the Coulomb force, gener-
ating a Hall voltage VHacross the device and a longitu-
dinal voltage Vxalong the device; however, here the Hall
voltage is no longer directly proportional to the mag-
netic flux density B.
74

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 6. Graph showing the Hall voltage VHand longitudinal voltage Vxfor a quantized Hall resistance device
in a magnetic flux density Band with constant current I. A diagram of such a device, pictured in the inset,
shows the alignment of the flux density perpendicular to the plane of the device.
Oscillations occur in the voltage Vxof Fig. 6 when the
applied current Iis kept constant and the flux density B
is varied. A series of constant-voltage plateaus arise in
the VHsignal for those regions of Bwhere Vxis small.
The transverse resistance, with a value of RH, is defined
as the ratio of the quantum Hall voltage VHacross the
device divided by the current Ithrough the device, and
thus has a constant value along a plateau. For example,
two of the plateaus in the figure have measured QHR
values of about 6 453.2 ⍀and 12 906.4 ⍀over a wide
range of magnetic flux densities.
Klaus von Klitzing discovered the integer quantum
Hall effect [58] during the night of February 4 and 5,
1980, at the High-Field Magnet Laboratory of the Max
Planck Institute in Grenoble, France. He knew immedi-
ately that this discovery had significance for resistance
metrology, because of his earlier experience as a sum-
mer student at the PTB. Von Klitzing contacted Volkmar
Kose at the PTB to arrange a calibration of his reference
resistors. The results, with a one-standard deviation un-
certainty of about 10⫺5RH, were announced at an inter-
national conference in June of 1980 and published [33]
that August. He won the Nobel Prize in physics in 1985.
Landwehr, his mentor, has written an account [59] of the
events leading up to the discovery.
It is experimentally observed that the plateaus have
the resistance values
RH=h/e2i, (3)
where iis an integer and h/e2is about 25 812.8 ⍀. The
i= 2, 3, and 4 plateaus are labeled in Fig. 6. Robert
Laughlin, a co-winner of the 1998 Nobel Prize in
physics for the development of an understanding of the
fractional QHE, provides an elemental derivation [60] of
Eq. (3).
This fundamental property arises because charges
moving in crossed magnetic and electric fields obey the
equation of motion, qvxBz=qEy, where qis the charge
and vxis the velocity in the direction perpendicular to
each field. Eycan be written as the derivative of the
electric potential Vywith respect to the y-direction,
which is the direction in which the Hall voltage is mea-
sured. Then,
冕Vmax
Vmin
dVy=冕ymax
ymin
vxBzdy. (4)
The integral on the left is just the Hall voltage, which is
the Hall resistance divided by the total current. With the
two-dimensional current density J=evxNSinser ted on
the right-hand side, the integration yields the equation
RH=B/eNS, where NSis the number density of conduct-
ing electrons within the 2DEG. This equation is a classi-
cal result, which predicts a linear relation between resis-
tance and the magnetic flux density B. In the QHE, near
75

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
the centers of the plateaus, all the allowed states are
filled and there is an energy gap to the next level. Equa-
tion (3) is found because of the following: (a) the elec-
trons in the 2DEG make cyclotron orbits about the mag-
netic flux lines and occupy states in a Landau level; (b)
the maximum number of states per unit area nsin a filled
Landau level is ns=eB /h; and (c) NS=in
s.
Workers in national metrology laboratories quickly
began studying the effect. The uncertainty in measuring
RHneeded to be reduced several orders of magnitude.
By August, 1980, M. E. Cage, B. F. Field, and R. F.
Dziuba from NBS were doing experiments [61] with R.
Wagner at the Naval Research Laboratory in Washing-
ton, DC, initially using a Bitter magnet until a 13 T
superconducting magnet was installed. Like von Klitz-
ing, they used silicon metal-oxide-semiconductor field-
effect transistors (Si MOSFETs) which have the unfor-
tunate weakness that static electricity can puncture the
oxide layer, rendering the device useless.
In 1982 D. Tsui and H. Stormer (1998 Nobel Prize
co-winners with Laughlin for the fractional quantum
Hall effect) invited NIST staff to Bell Labs in Murray
Hill, NJ. Their GaAs/AlGaAs heterojunction devices,
made by A. Gossard, were better than Si MOSFETs
because of the smaller effective masses of the electrons
in the 2DEG and wider plateaus. These devices, and
improvements in the measurement system, enabled the
first precision measurements of RH, with an uncertainty
of 2x10
⫺7RH[62].
Cage, Field, and Dziuba began monitoring the bank
of five 1 ⍀wire-wound resistors (whose average value
was the U.S. Legal Ohm) at NBS in terms of the quan-
tized Hall resistance RHusing an 8 T superconducting
magnet supplied by Bell Labs. In 1983, Tsui, Stormer,
and Gossard also gave NIST their best integer effect
GaAs/AlGaAs devices. The potentiometric QHR mea-
surement technique [63] and resistance scaling methods
were refined, allowing NBS to monitor the U.S. Legal
Ohm to within the fractional amount of 1.5 ⫻10⫺8[29].
Over a period of 5 years leading up to 1988, measure-
ments by Cage, C. T. Vandegrift, and Dziuba showed
that the U.S. Legal Ohm was decreasing by a fractional
amount of at least 5.3 ⫻10⫺8/year. The resistor-based
legal ohms in other countries were also drifting with
similar rates.
The relative uncertainties in the SI values of hand e
were about 1 ⫻10⫺6at that time, and so h/e2could not
be determined without absolute electrical standards
linked to the SI quantities. Therefore, at NIST, the best
SI value [28] of RHwas obtained using the calculable
capacitor chain (described in Sec. 2.3). The resulting SI
realization of RHhad an uncertainty of 2.4 ⫻10⫺8. The
NPL in the United Kingdom [64] and the NML in Aus-
tralia [65] also obtained SI values of RH, with relative
standard uncertainties of 6.7 ⫻10⫺8and 6.2 ⫻10⫺8, re-
spectively.
The CCE considered these and other data in recom-
mending the adoption of a new constant for maintaining
the ohm [57]. The 1990 exact, conventional value of the
von Klitzing constant,
RK⫺90 = 25 812.807 ⍀, (5)
was adopted by all NMIs on January 1, 1990, and be-
came the new basis for the U.S. Legal Ohm. The con-
ventional value was assumed by the CCE to have a
relative standard uncertainty of 0.2 ␮V/V. Again, by
convention, this uncertainty is not included in the uncer-
tainties of the representation of the ohm, since any offset
from the SI ohm will be consistent between different
laboratories using the QHE standard.
A set of technical guidelines [66] was also issued by
the CCE to assure reliable measurements of RHsince it
was found that the values can depend on parameters
such as the device temperature [67], the applied current
[68], and electrical contacts to the 2DEG [69]. It has
been demonstrated, when following the technical guide-
lines, that the value of RHis device-independent to a
fractional amount of no more than 3.5 ⫻10⫺10 [70, 71].
1.4.3 Ohm’s Law in Quantum Metrology
No deviation from the exact relations for the Joseph-
son and quantum Hall effects [Eqs. (1) and (3)] has been
proven either theoretically or experimentally, at least
when the conditions approach the ideal. A third and
complementary quantum physics effect is in develop-
ment at a number of laboratories, based on single elec-
tron tunneling (SET). At NIST, these SET devices can
be controlled so that an exact number of electrons can be
generated (using tuned bias voltages) in a certain period
of time for use in a measurement circuit. Proposals for
comparing the Josephson voltage to that of a QHR
device subjected to such an exact current [72, 73] are
under study at NIST and other NMIs. Ohm’slaw
(V=IR) could then be applied directly to help deter-
mine the fundamental physical constants eand h.
It is, however, difficult to develop high enough cur-
rents for such experiments, because the SET current of
a single device is limited by the capacitance and the time
constant of the device to about 1 pA [74]. In addition,
“charge offsets”[75] prevent large numbers of devices
from working simultaneously in the same circuit (due to
complexity in tuning the circuit), and it has not yet
proved possible to raise the current by running many of
the devices in parallel. This special limitation on current
so far has prevented any significant measurements using
SET currents in QHR devices. Instead, SET research
76

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
has led M. W. Keller, A. L. Eichenberger, J. M. Mar-
tinis, and N. M. Zimmerman at NIST to pursue a cryo-
genic capacitance standard [76] that is charged by a SET
device, for which only a small current is required.
2. Some Present-Day Electrical
Measurement Programs at NIST
2.1 DC Voltage
The volt is a derived unit in the SI, one definition of
which is the potential difference between two points on
a conductor carrying a current of one ampere when the
power dissipated is one watt. NIST maintains the repre-
sentation of the volt based on the Josephson effect, de-
scribed in Sec. 1.4 as a simple relationship between the
voltage across a superconductor—insulator—supercon-
ductor junction and the microwave frequency radiating
onto the junction. The relationship can be expressed by
the equation V=nf /KJ, where Vis the quantized voltage,
nis an integer, fis the microwave frequency, and
KJ=2e/his the Josephson constant determined by the
Plank constant hand the elementary charge e.
2.1.1 The NIST DC Voltage Standard Laboratory
The responsibilities for maintaining and disseminat-
ing the volt through NIST calibration services fall to
the Electricity Division’s Josephson voltage and
voltage-calibration laboratories. Two Josephson voltage
standard (JVS) systems, designated NIST-1 V and
NIST-10 V, operate as the U.S. representation of the SI
volt.TheNIST-1Vsystemusesa1VJosephson array
consisting of 2076 junctions in series. The NIST-10 V
system uses a 10 V array consisting of 20 208 junctions.
Figure 7 shows a picture of a 10 V Josephson array
made of niobium and Fig. 8 shows the zero-crossing
steps that each junction of the array can provide.
These two Josephson voltage standard systems are
compared every year by measuring a set of Zener
voltage standards to check the correctness of JVS oper-
ation. The difference between the two JVS measure-
ments at 1.018 V in the last two comparisons has been
less than 1 nV, with a combined expanded uncertainty of
less than 6 ⫻10⫺9(coverage factor k= 2) [77]. The
NIST-1 V system is dedicated to transfer the NIST rep-
resentation of the SI volt to a primary group of standard
cells on a regular basis. The NIST-10 V system is used
for evaluating new voltage standards and system soft-
ware, to develop a voltage measurement assurance
Fig. 7. This picture shows a Josephson junction array (18 mm ⫻9 mm) consisting of 20 208 junctions in series that
is able to provide voltages up to 12 V with ≈155 ␮V between two adjacent steps, when irradiated with microwave
radiation at a frequency of 75 GHz. Each Josephson junction provides zero-crossing voltage steps when microwave
radiation is applied through the finline antenna at the right.
77

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 8. Graph of zero-crossing voltage steps for a single Josephson
junction.
program and to participate in international and domestic
JVS intercomparisons.
A comparison between a portable BIPM JVS and the
NIST-1VJVSat1Vwascarried out in 1992 with a
relative agreement of 1.3 ⫻10⫺8(k= 2) for an indirect
comparison (via mutual measurements of an isolated
Zener voltage standard) and 3 ⫻10⫺10 (k=2)foradi-
rect comparison [78]. A recent comparison between the
BIPM and both NIST sites (Gaithersburg and Boulder)
involved measuring three transpor t Zener standards. The
fractional differences among the 10 V calibrations
traceable to these three JVS lie within 2.6 ⫻10⫺8with
a combined expanded uncertainty of 3.4 ⫻10⫺8(k=2)
[79].
Figure 9 describes the traceability of the dc voltage
calibration service to the SI. The NIST voltage calibra-
tion laboratory maintains a primary group of ten Weston
cells, housed in two separate enclosures with a temper-
ature stability of 10 ␮K. Because of the risk that a
standard cell could be damaged by a sudden change in
the voltage of the JVS, the cells are not calibrated di-
rectly against the JVS. Instead, the primary group of
standard cells is calibrated through a set of three Zener
transfer standards measured daily against NIST-10 V
and monthly against NIST-1 V, and is maintained with a
relative standard uncertainty of 3 ⫻10⫺8.
There are three measurement systems that perform
voltage calibrations, as shown in Fig. 9. Each of the
systems has a working cell group, calibrated daily
against the primary group of standard cells. All of the
NIST standard cell groups have better long-term pre-
dictability and lower medium and short-term noise com-
pared to Zener standards. The main external calibration
workload, containing customer standard cells and Zener
standards, is measured against working cell group 2800.
As a check, a Zener voltage standard is calibrated daily
against the working cell group 2800 through a resistive
ratio divider. This Zener standard is also calibrated
monthly against NIST-10 V to check the consistency of
the Zener calibrations, which are made through a resis-
tive ratio divider.
2.1.2 Industrial Needs in Voltage Metrology
Figure 10 shows the progression in NIST’s capability
in voltage metrology, compared with industry needs. In
the early 1960s, voltage measurements using a voltmeter
required a relative uncer tainty of several hundred times
10⫺6. Today, a high-end digital voltmeter is able to make
voltage measurements with a relative uncertainty of
three or four times 10⫺6. Meeting the needs of instru-
ment manufacturers, calibration laboratories, and mili-
tary laboratories requires a NIST voltage calibration ca-
pability with a relative uncertainty of a few times 10⫺7.
The NIST dc voltage calibration service described here
also supports NIST calibration services for high-preci-
sion digital multimeters and calibrators. The typical
turnaround time is about 3 weeks for Zener standards.
The time required can be several months for saturated
standard cells; this depends on how quickly the stan-
dards reach equilibrium after transport.
The term “intrinsic standard”is sometimes used to
describe a type of standard, such as a JVS, QHR stan-
dard, triple point cell, deadweight pressure gauge, etc.,
based on physical laws rather than on the stability of
physical artifacts which depend on bulk materials prop-
erties. There are approximately twenty industrial and
military calibration laboratories throughout the United
States that operate a JVS as a basis for traceable calibra-
tion measurements. The JVS consists of many cryogenic
and microwave components, and each of these, as well
as the environment and user technique, can contribute
uncertainty to the voltage measurement. Accordingly, it
is necessary to make intercomparisons among indepen-
dent JVS laboratories, to ensure the correctness of the
measurements in these laboratories, just as it is at the
international level. In 1991 NIST conducted the very
first JVS laboratory comparison experiment using
transportable 10 V Zener standards, in which five other
U.S. industrial and military laboratories participated
[80]. Such comparisons are now carried out regularly
under the auspices of the National Conference of Stan-
dards Laboratories, an industry trade association, with
support from NIST as necessary.
Most of the intercomparisons between Josephson
voltage standards use such a set of transport Zener stan-
dards. In most cases, the noise characteristics of the
Zener standards are the limiting factor in the uncer-
tainty. Moreover, Zener standards are subject to changes
due to environmental conditions of temperature, baro-
metric pressure, and relative humidity. The uncer tainty
78

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 9. Voltage calibration path at NIST, showing the Josephson voltage standard (JVS) 1 V system and 10 V system. Also shown are the
transfer Zener voltage references, primary standard cells, and three calibration systems. MAP standards are standard cells used for the
measurement assurance program.
of an intercomparison can be significantly improved if
corrections for the effects of these environmental condi-
tions are made. NIST is now developing a measurement
assurance program (MAP) [81] using a set of low noise
and well-characterized Zener standards to improve the
uncertainty of a MAP for JVS intercomparisons. It is
foreseeable that a relative standard uncertainty of
5⫻10⫺8(k= 2) or better for a MAP at 10(V level can be
achieved using this approach.
The challenge NIST is facing in the new millennium
is to meet even greater needs in industrial applications
and scientific research for more reliable, accurate and
economical voltage sources and for measurement tech-
niques. For example, a project to improve the present
JVS intercomparisons is currently under way. This en-
tails development of a compact JVS system that can be
shipped to another lab for a direct or indirect compari-
son with a second JVS. The compact JVS will enhance
the capability of the NIST calibration service for cus-
tomers who base their measurements on a JVS by offer-
ing them direct traceability at an uncer tainty level of a
few parts in 109.
79

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 10. Industry needs and NIST capabilities in voltage calibration.
The expanded uncertainty corresponds to a coverage factor k=2
(DVM: digital multimeter, JVS: Josephson voltage standard).
2.1.3 Future Developments in Voltage Metrology
When the CCE recommended international accep-
tance of the value of the Josephson constant KJ⫺90 =
483 597.9 GHz/V beginning Jan. 1, 1990, they believed
this provided a volt based on the Josephson effect that
agreed with the SI volt with a relative standard uncer-
tainty of 4 ⫻10⫺7. A recent moving-coil watt balance
experiment carried out at NIST, based on equating elec-
tric power and mechanical power, has determined the
Josephson constant to be 483 597.892 GHz/V, with a
relative standard uncertainty of 4.4 ⫻10⫺8[82]. The
agreement between the Josephson constant determined
in terms of the SI and the value of KJ⫺90 has thus been
improved by an order of magnitude. This result suggests
that the value of KJ⫺90 was well defined, and that there
is no need to make an adjustment of the Josephson
constant in the near future.
The progress in technology over the last thirty years
has made it possible to integrate tens of thousands of
Josephson junctions on a single chip and to generate
voltages up to 10 V. Now, there are active research and
development activities to manufacture more economical
and reliable Josephson voltage standards for better
voltage measurements. Results in high temperature su-
perconductor research may create a new family of
Josephson junctions usable at liquid nitrogen tempera-
ture (77 K) or at even higher temperatures. Progress in
cryogenics has encouraged manufacturers to develop
efficient cryo-coolers for cooling a conventional Joseph-
son array to its working temperature (4 K to 5.2 K),
saving the cost of liquid helium.
C. J. Burroughs, S. P. Benz, C. A. Hamilton, and T. E.
Harvey, working at the NIST Boulder laboratories, have
developed a new type of array with programmable
binary segments of Josephson junctions. The pro-
grammable array has step-amplitude a hundred times
larger than the conventional zero crossing steps [83]. To
improve the NIST voltage standard laboratory, such a
programmable array working at the1Vlevelwill be
tested as a backup to the NIST primary group of stan-
dard cells. This array might eventually replace the pri-
mary group of standard cells, thereby reducing the sys-
tem uncertainty by shor tening the steps in the voltage
dissemination chain. The noise immunity and high reso-
lution in voltage measurements provided by the pro-
grammable array now allows its use in the NIST watt
balance experiment, thereby reducing the uncertainty of
the voltage reference by a factor of eight. In the future,
an ultimate Ohm’slawor“quantum triangle”experi-
ment (combining the Josephson effect, the quantum
Hall effect, and the single-electron-tunneling device)
may be based on this type of JVS.
2.2 AC-DC Thermal Transfer Instruments
2.2.1 Development of the Thermal Converter at
NIST
AC voltages and currents in the frequency range from
low audio to hundreds of megahertz are measured at the
best uncertainties by comparison to dc standards with
ac-dc thermal transfer instruments. The field of ac-dc
thermal transfer metrology was essentially launched by
the publication of a paper nearly half a century ago
(1952) in the Journal of Research of the National Bu-
reau of Standards ,“Thermal Converters as AC-DC
Transfer Standards for Current and Voltage Measure-
ments”[84]. This paper by F. L. Hermach laid the
foundation for the techniques of ac-dc transfer and pro-
vided the theoretical basis for the ac-dc thermal transfer
structures. These devices were first used for accurate
calibration of ammeters and voltmeters in the audio fre-
quency range [84] and later at radio frequencies [85] for
measurements of voltage, current, and power. Hermach
and the staff of the NBS Electricity Division produced
important developments including the first transmis-
sion-line analysis of coaxial transfer standards [85].
In general, the rate of transformation of energy from
electrical to thermal form in thermal converters is pro-
portional to the root-mean-square (rms) values of cur-
rent and voltage. The heater temperature is a function of
the square of the heater current even if the constants in
the defining equation, covering the underlying physics,
vary with temperature or time. Since the response of
thermal converters is calibrated on direct current at the
time of use, ac-dc transfers are possible with little de-
crease in accuracy from drift or external temperature
influences.
80

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Traditional thermal converters contain wire heaters or
thin, metal heater structures. The temperature of the
heater is typically monitored with one or more thermo-
couples, also made of wire or thin metal film. The
best-performing primary standards usually contain
many thermocouples in an arrangement that minimizes
ac-dc difference by reducing both heater temperature
and thermal gradients. Current research at NIST in-
cludes two areas directed at new thermal converters
suitable for both primary and working standards.
2.2.2 Thin-film Multijunction Thermal Converters
Multijunction thermal converters (MJTCs) are used
in very high-accuracy ac-dc difference metrology be-
cause they have very small ac-dc differences, follow the
rms law of excitation, and produce high output emfs.
MJTCs traditionally have been fabricated from wire
heater resistors and thermocouples. The project to de-
velop thin-film MJTCs (FMJTCs) involves the use of
micro-machining of silicon and photo-lithography on
thin films to produce high-performance thermal transfer
standards. Multi-layer FMJTCs have been designed, fab-
ricated, and tested at NIST by J. R. Kinard, D. B.
Novotny, and D. X. Huang, and new improved convert-
ers are under development [87].
The basic elements of the devices are a thin-film
heater on a thin dielectric membrane, a silicon frame
surrounding and supporting the structure, and thin-film
thermocouples positioned with hot junctions near the
heater and cold junctions over the silicon. Carefully se-
lected materials in new thermal designs are required,
along with very accurate dimensioning of the heater and
thermocouples. The heater and thermocouples are sput-
ter deposited and patterned with photolithography. Con-
tributions to ac-dc difference from the Thomson effect
and other effects are further reduced by the appropriate
choice of heater alloy. Figure 11 shows a cross section
of an FMJTC.
Integrated micropotentiometers are thermal transfer
devices that contain FMJTCs and thin-film output resis-
tors fabricated as an integrated structure on the same
silicon chip. Figure 12 shows an integrated micropoten-
tiometer including the FMJTC structure. New versions
of the FMJTCs and integrated micropotentiometers are
under development that include new membrane materi-
als and vacuum packaging, with the help of novel etch-
ing techniques such as front and back surface etching.
2.2.3 Cryogenic Thermal Converter
At audio frequency, thermal and thermoelectric ef-
fects ultimately limit the measurement uncertainty in
conventional room-temperature thermal conver ters.
Heater powers as high as a few tens of milliwatts and
temperature differences as high as 100 K are common in
some thermal converters. To reduce these effects and to
achieve very high temperature sensitivity, a novel sensor
employing a superconducting resistive-transition edge
thermometer is being developed at NIST by C. D.
Reintsema, E. N. Grossman, J. A. Koch, J. R. Kinard,
and T. E. Lipe [88, 89]. Since the new conver ter oper-
ates at temperatures below 10 K and is mounted on a
platform with precise temperature control and very
small temperature gradients, the thermal and ther-
moelectric errors are potentially quite small. Because of
the very high temperature sensitivity of the supercon-
ducting transition, this converter also offers the possi-
bility of direct thermal transfer measurements at very
low signal levels.
Fig. 11. Cross section of a thin-film multijunction thermal converter.
81

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 12. Integrated micropotentiometer including the thin-film multijunction thermal converter (FMJTC) structure.
Figure 13 shows the experimental platform for the
prototype cryogenic thermal converter. This transfer
standard consists of a signal heater, trim heater, and
temperature sensor all mounted on a temperature-stabi-
lized platform. The sensor resistance is measured by an
ac resistance bridge, and the temperature of the assem-
bly is held constant by the closed loop application of
power to the trim heater. A NbTa thin-film meander line
is used as the thermal sensor and is thermally biased to
operate within its superconducting-resistive transition
region. The signal heater in the prototype device is a 7
⍀thin-film meander line and the trim heater is a 450 ⍀
PdAu thin-film meander line, both adjacent to the detec-
tor on the silicon substrate. To ensure temperature sta-
bility, the entire converter assembly is mounted on a
second platform controlled at a slightly lower tempera-
ture. This intermediate stage is thermally isolated and is
controlled by a second ac resistance bridge using an-
other transition edge sensor and heater.
Using this new cryogenic conver ter, measurements
have been made at signal power levels of a microwatt,
which is around 1000 times lower than is possible with
room-temperature converters. Characterization using a
fast-reversed-dc source has shown that the thermoelec-
tric errors are presently in the 1 ␮V/V to 2 ␮V/V range.
These early results are encouraging, but considerable
improvement both in the resistance bridge performance
and in the input transmission line will be necessary for
this new device to be a candidate for consideration as a
primary standard.
2.3 Impedance Measurements
2.3.1 Development of the Calculable Capacitor at
NIST
The link between the QHR standard and SI units was
derived from modern experiments, including primarily
the NIST calculable capacitor and the calculable capac-
itor chain. The calculable capacitor experiment is simi-
lar, in the degree of importance in modern electrical
metrology, to the Josephson and quantum Hall effects.
However, only a few NMIs maintain calculable capaci-
tor experiments, while dozens now maintain JVS and
QHR measurement systems.
82

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 13. Photograph of a cryogenic thermal transfer standard showing the chip, converter stage, and reference platform. The cryostat base-plate
is at 4.2 K, the reference platform is at 6 K, and the converter stage is at 6 K + ⌬Tc.
Thompson and Lampard [90] first published the the-
ory behind the calculable capacitor in 1956. The equa-
tion below shows that the two cross capacitances C1and
C2for a specially designed cylindrical cross capacitor
can be found if the length of the electrodes Lis known
(
␧
0is the electric constant):
exp冉⫺␲ C1
L
␧
0冊+exp
冉⫺␲ C2
L
␧
0冊= 1 (6)
The first report of a measurement based on a calculable
cross-capacitor at NIST was in 1961 by Cutkosky [13].
In addition to measurements of the NIST primary stan-
dard of capacitance, this effort entailed the use of a
special quadrature bridge by which a resistor with a
known frequency response could be measured, first rel-
ative to a capacitor, and then used to measure resistance
standards. In this fashion, Cutkosky obtained the value
of the U.S. Legal Ohm with a relative standard uncer-
tainty of 2.1 ⫻10⫺6using horizontally-mounted cylin-
drical gauge bars as capacitor electrodes. Around the
same time, Clothier at the National Measurement Labo-
ratory in Australia constructed a calculable capacitor
consisting of four vertical electrodes [91]. This was
used, starting in 1963, in Australian experiments to
realize the (SI) farad and (SI) ohm with a relative stan-
dard uncertainty of less than 1 ⫻10⫺7.
After the success of Clothier’s calculable capacitor
design, a new cross-capacitor was constructed at NIST
in the late 1960s utilizing a similar geometry. This ca-
pacitor is the one used at NIST today, although with
several improvements. In 1974, Cutkosky reported the
first SI value of the U.S. Legal Farad and the U.S. Legal
Ohm [92] derived from this calculable capacitor. At the
time, the national units of impedance were defined as
the average value of a bank of 10 pF fused silica capac-
itors [93] in the case of capacitance and a bank of 1 ⍀
Thomas-type resistors for resistance. The realization of
the ohm from the calculable capacitor at NBS in terms
of the SI units of length and time was evaluated to have
83

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
a relative standard uncertainty of 0.03 ⫻10⫺6. Shields
et al. [28] reported the second NIST realization of the
ohm and farad in 1989 using the same system after
making several improvements. The relative uncertainties
were 0.022 ⫻10⫺6for the U.S. Legal Ohm and
0.014 ⫻10⫺6for the U.S. Legal Farad. Jeffery et al.
reported [94] the most recent realization of the ohm in
1996 with a relative uncertainty of 0.024 ⫻10⫺6.
2.3.2 The NIST Calculable Capacitor
The calculable capacitor consists of four vertical
cylindrical bars arranged at the corners of a square in
the X-Yplane, placed symmetrically about the central
Z-axis (see Fig. 14). Capacitance is measured between
diagonal pairs of opposite bars, which for this geometry
gives two capacitance values C1'≈C2'(each per unit of
Fig. 14. Diagram of the calculable capacitor electrodes, with one of the four main electrodes shown in cut-away
view. Measurement voltages are applied across opposite pairs of main electrodes, and the central blocking
electrode can be moved vertically to change the capacitance.
84

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
length). The Thompson-Lampard equation [Eq. (6)] re-
duces to C' ≈2 pF/m, where
C' =(C'
1+C'
2)/2. (7)
Second-order terms due to the difference between C1'
and C2'contribute less than 10⫺9C' to this capacitance
in the NIST calculable capacitor.
The working length of the calculable capacitor is
defined by two cylindrical electrodes on the central
Z-axis. Except in the space between the ends of these
electrodes, the electric field between opposite capacitor
bars is completely blocked. A change in the vertical
position of either of these grounded electrodes effec-
tively changes the length associated with C1'and C2'.In
practice, the lower blocking electrode is fixed and the
upper one is moved to adjust the capacitor’s value. Mea-
surements are made by comparing a fixed-value 10 pF
capacitor to the calculable capacitor at two positions of
the moveable electrode, where the values are 0.2 pF and
0.7 pF. Displacement of the electrode between these two
positions yields a difference of 0.5 pF. By measuring the
displacement of the blocking electrode rather than the
absolute length of the capacitor, many problems associ-
ated with fringing effects at the ends of the capacitor are
eliminated.
A Fabry-Perot interferometer measures the relative
displacement of optical flats mounted in the moveable
and fixed blocking electrodes. The Fabry-Perot interfer-
ometer employs a fringe-locking laser optical system.
The system is enclosed in a metal case, which is kept
under vacuum. This eliminates the need to apply correc-
tions due to the dielectric constant of air and provides a
clean environment for the electrodes. The calculable
capacitor apparatus is shown in Fig. 15. From the Fabry-
Perot length measurement and Eq. (6), the value of the
calculable capacitor is determined and a value is as-
signed to the fixed capacitor through comparison in an
ac capacitance bridge [92].
Fig. 15. Calculable capacitor (vacuum chamber at center) and measurement apparatus.
85

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
The present relative combined standard uncertainty
for this measurement [95] is 0.019 ⫻10⫺6and the
sources of this uncertainty are listed in Table 1. The
largest relative standard uncertainty in Table 1 is that
due to geometric imperfections. This includes the rela-
tive alignment of the axes of the bars to each other and
to the blocking electrodes, alignment of the electrical
axis of the capacitor and the optical axis of the interfer-
ometer, and imperfections in the bars. The magnitude of
the uncertainty attributed to geometrical imperfections
is the standard deviation of measurements of 0.1 pF
increments made along the length of the capacitor. The
blocking electrode end is presently a cylindrical spike,
but recent tests by Jeffery [96] have shown that a mod-
ified cone shape (cone with a very short cylindrical
spike) could reduce the effect of geometric imperfec-
tions in the bars.
2.3.3 Realization of the SI Ohm: The Calculable
Capacitor Chain
Since 1990, the U.S. Legal Ohm has been defined by
the internationally agreed upon value of the i= 1 resis-
tance plateau of the QHR, RK⫺90. The link between the
calculable capacitor and the ohm is made through a
sequence of measurements called the calculable capaci-
tor chain. This sequence is shown in Fig. 16. Measure-
ment of the QHR via the calculable capacitor provides
one of the best determinations, expressed in terms of SI
units, of the von Klitzing constant RKand the fine-struc-
ture constant
␣
[57, 97, 98]. The inverse fine-structure
constant
␣
⫺1can be obtained from the measured value
of RKand Eq. (6) with no additional uncertainty since it
is generally accepted that RKis related to the fine-struc-
ture constant
␣
by
RK=h/e2=
␮
0c/2
␣
, (8)
where his the Planck constant and eis the elementary
charge. The magnetic constant (permeability of vac-
uum),
␮
0=4␲⫻10⫺7N/A2, and the speed of light in
vacuum, c= 299 792 458 m/s, are exactly defined in the
SI.
The 1989 NIST value of the von Klitzing constant
is RK= 25 812.807 23(61) ⍀, and the 1996 value is
RK= 25 812.808 31(62) ⍀, which is larger by a frac-
tional amount of 4.2 ⫻10⫺8. The difference in the two
results is small but significant, and we believe that the
most recent result is more reliable as it is based on a
series of measurements and not on one measurement as
was the 1989 result. From our most recent determina-
tion, we find that
␣
⫺1= 137.036 003 7(33). Both of the
above are in close agreement with recent quantum elec-
trodynamic (QED) calculations of the anomalous mag-
netic moment of the electron aeby Kinoshita [99],
which may be combined with an accurate experimental
value of aeto derive a. This combined experimental-
theoretical assignment, a⫺1= 137.035 999 58(52) is
3.0 ⫻10⫺8smaller than the 1996 value NIST reported.
Table 1. Relative standard uncertainties in the measurement of the 10 pF bank with the calculable
capacitor. The last row is the root-sum-square (rss) of the uncertainties listed in the rows above.
Source of uncertainty Relative standard uncertainty
(i.e., estimated relative standard
deviation)
Type A standard uncertainties
Variability of repeated observations 2 ⫻10⫺9
Type B standard uncertainties
Geometrical imperfections in the calculable capacitor 15 ⫻10⫺9
Laser and interferometer alignment 3 ⫻10⫺9
Frequency (loading) corrections 4 ⫻10⫺9
Microphonic coupling 5 ⫻10⫺9
Voltage dependence 5 ⫻10⫺9
Transformer ratio measurement 2 ⫻10⫺9
Bridge linearity and phase adjustment 3 ⫻10⫺9
Detector uncertainties 2 ⫻10⫺9
Drift between calibrations and failure to close 6 ⫻10⫺9
Coaxial choke effectiveness 1 ⫻10⫺9
Temperature corrections for 10 pF capacitors 2 ⫻10⫺9
Relative standard uncertainty (rss) 19 ⫻10⫺9
86

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 16. The calculable capacitor measurement chain. Capacitance
and resistance standards are represented by red boxes, with QHR
representing the quantized Hall resistance. Measurement bridges used
to relate the standards to each other are represented by ovals.
2.3.4 Maintenance of the Capacitance Unit from
the Calculable Capacitor
Including all the preliminary calibration measure-
ments, the calculable capacitor realization of the farad
requires on the order of 1 month to complete. A 10 pF
reference capacitor, measured against the calculable ca-
pacitor, is taken to another laboratory and the unit trans-
ferred to a 10 pF reference bank of four fused silica
capacitors, which are the representation of the unit be-
tween calculable capacitor measurements.
The NIST 10 pF capacitor reference bank capacitors
are intercompared weekly, along with several other ca-
pacitors of the same type, to monitor their behavior over
time. The 10 pF fused silica capacitors were developed
at NIST by R. D. Cutkosky and L. H. Lee [93], and have
a very low average fractional drift rate of 2 ⫻10⫺8per
year. One of the 10 pF fused silica capacitors is shown
in Fig. 17. These capacitors are sensitive to dimensional
changes, and have a temperature coefficient of 10 ␮F/F
per ⬚C. Thus, they are kept in a temperature-controlled
oil bath at (25 ⫾0.0001) ⬚C. Inside each standard, a
calibrated copper resistor in close thermal contact with
the capacitance element allows the temperature of the
capacitor to be recorded when the capacitor’s value is
measured in order to correct the capacitance value to a
selected reference temperature.
2.3.5 Impedance Calibration Laboratory
The NIST impedance calibration laboratory (ICL)
disseminates the SI units (the farad and henry) through
capacitance and inductance calibrations for customers,
both inside and outside of NIST. The ICL provides cal-
ibrations of nominal-valued capacitors in the range from
0.001 pF to 1 ␮F in the frequency range from 100 Hz
to 10 kHz. Customers include aerospace companies,
instrumentation companies, the U.S. armed forces, sec-
ondary calibration laboratories, and other U.S. and for-
eign national laboratories. The laboratory also provides
capacitance calibrations at 1 kHz that are used by the
high-frequency calibration laboratories at NIST for their
calibrations at frequencies above 1 MHz. The ICL pro-
vides calibrations of inductors in the range 50 mH to
10 H in the frequency range from 65 Hz to 10 kHz. The
inductance value is found using the Maxwell-Wien
bridge [100], which derives the value of the inductor by
comparison against two resistors and a capacitor.
2.3.6 Extension of Measurement Frequency
One of the main areas of research in impedance is the
extension of capacitance measurements to other fre-
quencies besides 1592 Hz (
␻
=10
4), the designed oper-
ating frequency of the calculable capacitor. This fre-
quency was chosen to allow the calculable capacitor
chain to transfer the impedance value of a 1000 pF
capacitor to that of a 100 k⍀resistor.
87

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 17. A 10 pF fused-silica capacitance standard.
Presently the primary laboratory operates only at
1592 Hz. The capacitance unit is transferred from the
calculable capacitor to the ICL via fused silica dielectric
capacitors whose value has been determined only at
1592 Hz. However, the ICL performs customer calibra-
tions at 10 kHz, 1000 Hz, 400 Hz, and 100 Hz and the
frequency dependence of the fused silica capacitors has
a relatively large uncertainty. NIST plans to develop
multi-frequency measurement capabilities to better sup-
port our customers needs in the frequency range from
100 Hz to 10 kHz. This will require design and con-
struction of capacitance bridges that work at multiple
frequencies as well as an evaluation of the calculable
capacitor system at these frequencies.
2.3.7 AC QHR Measurements
The calculable capacitor is one of the most direct
ways to obtain the SI farad and allows the realization of
the SI ohm. However, it is a very difficult and resource
intensive experiment; thus only a few NMIs in the world
have implemented it. Typical set-up times are from five
to ten years including precision machining and detailed
evaluation of the system. Furthermore, the present cal-
culable capacitor uncertainty is close to its expected
limit due to the macroscopic nature of the experiment.
There has been much interest in finding a way to obtain
a capacitance unit by other means. An ac determination
of impedance based on the QHR is one of these alterna-
tives.
Since most NMIs already have a dc QHR system for
resistance calibrations, it would be very convenient if the
quantum Hall effect could be used for impedance deter-
minations as well. This would be a reversal of the chain
of measurements that is used to realize the SI ohm
without the ac/dc conversion step (See Fig. 16). If the
QHR could be used directly with a quadrature bridge,
which relates capacitance to resistance, this chain could
88

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
be made even shorter. Several national laboratories
[101, 102, 103, 104, 105] have begun working on ac
QHR measurements with the hope that the ac QHR
could provide a working unit similar to the internation-
ally agreed upon value of the dc QHR.
Several difficulties have been encountered with the ac
QHR measurements, so far producing a much higher
limit on the uncertainty than with the dc QHR. These
include an observed linear frequency dependence and
differences from the dc value of about 1 ⫻10⫺7. NIST
has begun to develop an ac QHR measurement system.
As one of the few laboratories in the world able to link
the dc QHR to the calculable capacitor, NIST will be
able to compare the ac and dc QHR measurement chains
as well as determine SI values of the ac QHR.
2.4 DC Resistance
2.4.1 Quantized Hall Resistance Measurements
Shortly after the discovery of the QHE, NBS devel-
oped a system based on the QHE to monitor the U.S.
Legal Ohm, maintained by five Thomas-type resistance
standards, with a relative uncertainty of a few times 10⫺8
[106]. This consisted of a constant current source, a
potentiometer, and an electronic detector. The current
source energized the QHE device and a series-con-
nected reference resistor of nominal value equal to the
QHR. With the potentiometer balancing out the nominal
voltage across either resistance, the detector measured
the small voltage difference between the QHE device
and reference resistor. Scaling down to the 1 ⍀level was
accomplished using specially-constructed Hamon trans-
fer standards.
Starting in 1991, a QHR laboratory was set up near
the resistance calibration laboratory for the routine
maintenance of the U.S. legal ohm. This laboratory uses
a cryogenic current comparator (CCC) resistance bridge
[107], which in a two-step process can compare the
QHRtoa1⍀resistor. The QHR device is mounted on
a special holder inserted in a 4He cryostat containing a
superconducting magnet and a 3He refrigeration system.
Magnetic fields up to 16 T and temperatures as low as
0.3 K are achievable with this system. The QHR plateaus
that are measured have resistances of 6453.2 ⍀or
12 906.4 ⍀with currents of 20 ␮Ato60␮A f lowing
through the device. The QHR device is measured
against a bank of five 100 ⍀resistors, immersed in an
oil bath maintained at (25.000 ⫾0.003) ⬚C using a CCC
bridge as shown in Fig. 18.
The NIST CCC devices developed by R. F. Dziuba
and R. E. Elmquist are of the overlapped-tube type [108]
with a commercial SQUID sensor to detect the ampere-
Fig. 18. Schematic of a cryogenic current comparator resistance
bridge.
turn balance condition of the comparator. In order to
eliminate leakage currents, the current sources are float-
ing and optically isolated from one another [109]. A
commercial nanovolt detector, D (see Fig. 18), senses
the voltage difference across the resistors, and provides
a feedback current through Rfand Nf. The feedback
current is monitored by measuring the voltage drop
across Rfwith an optically isolated digital voltmeter and
is a measure of the difference of the resistor corrections.
The QHR measurements are carried out only a few
times a year, while the scaling measurements are done
more frequently to monitor the banks of resistors used
for customer calibrations. In 1999, a comparison at
NIST between a transportable QHR system of the BIPM
and the NIST QHR system agreed to within a combined
relative standard uncertainty of 2 ⫻10⫺9, for similar
measurements of a 100 ⍀standard [110].
2.4.2 Standard Resistors
Since January 1, 1990, the maintenance of the U.S.
legal ohm has been based officially on the QHE. How-
ever, the complexity of the experiment and “odd-value”
resistance of the QHR does not make it practical for the
routine support of resistance measurements where com-
parisons are normally made on standard resistors of
nominal decade values. Therefore, banks of 1 ⍀, 100 ⍀,
and10k⍀standard resistors maintain the ohm between
QHR measurements.
89

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
2.4.2.1 1 ⍀Resistors
Thomas-type 1 ⍀resistors have been used for over 50
years to maintain the laboratory value of the ohm in
many NMIs, including NIST. The NIST 1 ⍀bank con-
sists of five Thomas resistors constructed in 1933 [10];
these are among a number of the original standards built
by J. Thomas that are still in use. The high stability of
the Thomas resistor is due to the thorough anneal at
550 ⬚C, and the temperature coefficient of resistance
(TCR) has been reduced to close to zero at a tempera-
ture between 20 ⬚Cand30⬚C by proper heat treatment.
However, the Thomas resistors need to be operated in a
controlled environment (oil bath) because the curvature
of its temperature vs. resistance curve over this range is
approximately ⫺0.5 ⫻10⫺6/K2. The resistors are sealed
in dry air in the annular space between two coaxial
cylinders because the wire is subject to surface oxida-
tion and has a significant pressure coefficient of resis-
tance (PCR). Figure 19 indicates the drift of the 1 ⍀
bank prior to the re-definition of the ohm in 1990. Until
recently, the Thomas-type resistors were available com-
mercially.
2.4.2.2 100 ⍀and10k⍀Standard Resistors
Both the 100 ⍀and10k⍀banks of resistors consist
of commercial standard resistors hermetically sealed in
oil-filled containers. Each standard contains ten 1 k⍀
resistors constructed of Evanohm1wire wound on mica
cards and connected in parallel for the 100 ⍀standards
and in series for the 10 k⍀standards. The 100 ⍀refer-
ence bank consists of five standards housed in an oil
bath controlled at a temperature of (25.000 ⫾0.003) ⬚C.
The 10 k⍀reference bank contains two standards main-
tained in a laboratory environment at a temperature of
(23.0 ⫾0.5) ⬚C.
2.4.2.3 High-Value Standard Resistors
In 1996, R. F. Dziuba and D. J. Jarrett developed a
process for fabricating stable, transportable, high-value
standard resistors of decade nominal values from 1 G⍀
to 10 T⍀[111]. The resistance elements of these stan-
dards consist of precious-metal-oxide (PMO) film resis-
tors that are commercially available. To improve their
stability, these PMO film resistors are pre-aged by exter-
nal heating. For the 1 G⍀,10G⍀and 100 G⍀stan-
dards, selected resistors are mounted in thick-walled
1Certain commercial equipment, instruments, or materials are
identified in this paper to foster understanding. Such identification
does not imply recommendation or endorsement by the National
Institute of Standards and Technology, nor does it imply that the
materials or equipment identified are necessarily the best available for
the purpose.
Fig. 19. Time dependence of the U.S. ohm representation.
90

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
brass cylinders using end plates with glass-to-metal
seals (as shown in Fig. 20). These containers are purged
with dry nitrogen gas, hermetically sealed, and shock
mounted in aluminum enclosures. The shock-mounting
technique reduces effects due to transport. Specially
designed metal-insulator-metal containers are used to
seal the 1 T⍀and10T⍀standards. This design allows
the metallic end fittings to be driven at separate guard
potentials nominally equal to the potentials at the resis-
tor terminations and greatly suppresses leakage currents
flowing across the glass insulator of the seals. Signifi-
cant improvement in stability and the elimination of
humidity and pressure effects is achieved by pre-aging
and hermetically sealing the PMO film resistors.
2.4.3 Techniques Used for Resistance Calibrations
The best evidence indicates that a Wheatstone bridge
manufactured by the Otto Wolff firm in Berlin was used
to compare resistance standards during the early years of
NIST. It included a method for extending the resistance
range by 10:1 ratios. F. Wenner designed a bridge for
comparing resistors that was built in 1918 and continued
in service for over 50 years [112]. It was a combination
bridge that could be used as a simple Wheatstone bridge
or Kelvin double bridge with a 1:1 or 10:1 ratio. In 1969,
a dc current comparator bridge [113] replaced the Wen-
ner bridge for the comparison of Thomas-type 1 ⍀
resistance standards. This dc current comparator mea-
surement system was automated in 1982 and is still in
use today for calibrating customer resistors. Scaling to
higher resistance decades was achieved through the use
of Hamon transfer standards having 10:1 and 100:1
ratios [114].
NIST provides a calibration service for standard resis-
tors of nominal decade values from 10⫺4⍀to 1014 ⍀.To
achieve low uncertainties, eight measurement systems
have been developed that are optimized for the various
resistance levels [115]. Over the years from 1982 to
1997, six of the systems, covering the full 19 decades of
resistance, have been automated. The main methods of
comparing standard resistors for NIST calibrations uti-
lize direct current comparator (DCC) bridges and resis-
tance-ratio bridges.
2.4.3.1 Direct Current Comparator Bridges
The DCC, because of its insensitivity to lead resis-
tances, high level of resolution, and excellent ratio lin-
earity and ratio stability, is used to measure four-termi-
nal standard resistors at the low end of the resistance
range from 100 ␮⍀ to 100 ⍀. For example, Thomas-
type resistors (five comprising the reference bank, along
with two check standards and eight resistors under test)
are connected in series in the primary circuit of one
automated DCC bridge [116]. The value of an unknown
or check standard can be determined by indirectly com-
paring its voltage drop to the mean of the voltage drops
of the reference bank via a stable 0.5 ⍀resistor in the
secondary circuit of the DCC. The relative standard
uncertainty of this measurement system is estimated to
be 0.022 ⫻10⫺6.
2.4.3.2 10 k⍀Measurement System Resistance-
Ratio Bridge
Many industrial standards laboratories maintain their
primary reference standard of resistance at 10 k⍀, near
the middle of the resistance range; consequently, this
resistance level constitutes a significant portion of the
calibration workload at NIST. To improve these mea-
surements an automated guarded system for the com-
parison of 10 k⍀standard resistors was developed
[117]. The measurement system is based on the War-
shawsky bridge, which includes auxiliary or fan resis-
tors at the branch points of the bridge to eliminate
first-order errors caused by lead resistances. The auto-
matic selection of resistors is achieved by a unique,
Fig. 20. Hermetically-sealed resistor container assembly with glass-to-metal seals and copper purging tubes
soldered to end plates.
91

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
programmable, guarded coaxial-connector panel. A
computer-controlled XYZ positioning system (shown in
Fig. 21) is used to move a four-connector Z-axis panel
(connected to the bridge) over a panel of 72 coaxial
connectors mounted in the XY plane. This provides for
18 independent four-terminal channels. The combined
relative standard uncertainty of this measurement sys-
tem is 0.02 ⫻10⫺6.
2.4.3.4 1 k⍀to1M⍀Measurement System
Resistance-Ratio Bridge
An automated measurement system, based on the un-
balanced-bridge technique, was developed in 1989 to
replace the manual system used to measure resistors in
the range 1 k⍀to1M⍀[115]. This technique is re-
ferred to as the “ring method”since the resistors are
connected in a ring configuration as shown in Fig. 22.
The system is designed to measure the differences
among six nominally-equal, four-terminal standard re-
sistors of the Rosa type that are mounted on a stand
located in a temperature-controlled oil bath, but it has
the flexibility to accommodate resistors operating in the
laboratory air environment. In operation a voltage is
applied across opposite corners of the ring (A and A'),
which divides the ring into two paralleling branches
each containing three resistors. Then, a DVM measures
voltages between opposite potential terminals of the
resistors (V1through V6) for the two directions of cur-
rent. To complete the sequence of measurements, the
applied voltage points are rotated in a clockwise or
counterclockwise direction to each other pair of resistor
Fig. 21. Photograph of the programmable guarded coaxial switching system. The robotic xyz translation stage used for switching has been labeled
“Jake”.
92

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 22. Connection of resistors for measurements by the ring
method, showing the three subsets of connections for voltage measure-
ments.
connection points (B and B') and (C and C'). Again
voltage measurements are taken between corresponding
terminals of the resistors that are at nearly equal poten-
tials. From the three subsets of voltage measurements,
one obtains a set of nine linear equations that can be
solved using a least-squares technique. Values of the
resistors can be calculated if the value of at least one of
the resistors in the ring is known. The system is operated
with two reference standards, one check standard, and
three unknowns.
2.4.3.5 10 M⍀to 100 T⍀Measurement System
In 1996, an automated guarded bridge was developed
for calibrating multimegohm standard resistors from
10 M⍀to 100 T⍀[118]. This innovative bridge differs
from the conventional Wheatstone bridge in that two of
the ratio arms are replaced by programmable voltage
sources. The low output impedances of the voltage
sources along with the active guard network reduce er-
rors caused by leakage currents. As shown in Fig. 23, Rx
and Rsare the unknown and standard resistors with their
respective guard resistors rxand rs. Multiple ratios up to
1000/1 can be selected by adjusting the outputs of the
voltage sources. An electrometer with a resolution of
Fig. 23. Schematic of a guarded multimegohm resistance bridge,
used for comparing standard resistors up to 100 T⍀.
⫾3 fA in the current mode is used as the detector. The
ratio of the guard resistors rx/rsis nominally equal to
Rx/Rsand at balance, Rx=RsV1/V2.
2.4.4 Resistance Scaling
NIST maintains a bank of reference standard resistors
at each decade level of resistance. An unknown standard
resistor is indirectly compared to a reference bank of the
same nominal value using the substitution technique,
where the unknown and reference resistors are sequen-
tially substituted in the same position of a bridge circuit.
This technique tends to cancel errors caused by ratio
non-linearity, leakage currents, and lead and contact re-
sistances. To verify that the values of the reference
banks are consistent with the QHR, scaling measure-
ments are completed periodically proceeding from the
1⍀, 100 ⍀or 10 k⍀banks, whose values are based on
recent QHR determinations, to the other reference
banks. The up or down scaling is done in steps of 10 or
100, using either a CCC bridge, Hamon transfer stan-
dards, or DCC bridge.
2.4.4.1 CCC Bridge Scaling
Periodically, a CCC bridge in the calibration labora-
tory is used to intercompare the 1 ⍀, 100 ⍀, and 10 k⍀
reference banks to verify the consistency of their pre-
dicted values based on previous QHR determinations.
Also, these banks are checked against a 1 ⍀standard
resistor and 100 ⍀reference bank in the QHR labora-
93

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
tory. All measurements are done with the resistors in
situ by means of shielded cables between oil baths in the
two laboratories. The automated CCC bridge system is
similar to those described in Sec. 2.4.1 with a combined
relative standard uncertainty of 0.005 ⫻10⫺6.
2.4.4.2 Hamon Scaling
Hamon transfer standards provide accurate ratios of
10/1 and 100/1 for extending the resistance range in
multiple decade values from 10 k⍀to 100 T⍀. The
main advantage of these transfer standards is that they
are calibrated at one resistance level and are then used
with equal accuracy at different resistance levels as
short-term reference standards. Typically, the Hamon
device contains ten nominally-equal resistors perma-
nently connected in series by means of “tetrahedral”
junctions. Each junction has two current and two poten-
tial terminations, and the four-terminal resistance of
each junction is adjusted to be negligible compared to
the resistance of a main resistor. The transfer standard
can be connected with the ten resistors in a parallel
mode using special fixtures. The Hamon transfer stan-
dard can also be connected in a series-parallel configu-
ration to establish an accurate 10/1 ratio. Hamon-type
resistance scaling from 1 ⍀to 100 ⍀and from 100 ⍀
to 10 k⍀were checked against measurements with a
CCC bridge and agreement was within a fractional
amount of about 0.01 ⫻10⫺6, the practical limit of ac-
curacy using Hamon transfer standards with conven-
tional resistance bridges.
2.4.4.3 DCC Scaling
It is difficult to construct accurate Hamon transfer
standards with main resistances of less than 10 ⍀. The
major difficulties are with the adjustment and stability
of the resistances associated with the tetrahedral junc-
tions and fan resistors. Therefore, to extend the resis-
tance scale below 1 ⍀, NIST uses DCC resistance
bridges with ratios of 1/10, 1/100, and 1/1000. The ratio
accuracy and linearity of a DCC is self-checking with a
resolution of better than 0.01 ⫻10⫺6. The ratio accu-
racy can also be checked using two higher-valued stan-
dard resistors whose values are based on CCC or Hamon
scaling techniques.
3. Electrical Metrology at NIST in the
Twenty-first Century
The four metrological areas we have described in the
last section provide NIST customers with a sound basis
for measurements of voltage, impedance, and through
Ohm’s law, of current. The quantum-effect standards on
which the volt and ohm are now based, the thermal
voltage converter, and the calculable capacitor are some
of the most fundamental advances of modern metrology.
There have also been innumerable incremental advances
in electrical metrology over the past hundred years,
which have brought about changes that are no less sig-
nificant. These include transportable reference stan-
dards used in international comparisons, which can
maintain voltage and resistance values to within a frac-
tional amount of a few times 10⫺8, digital meters with
remarkably stable and linear measurement capabilities,
and better techniques for measuring the fundamental
electrical and mechanical quantities of power and en-
ergy. The 21st century may also provide startling new
physics and metrology, and metrologists at NIST are
continuing to redesign and improve the structure of ba-
sic electrical measurement standards. Meanwhile, tech-
nology is changing rapidly and improvements in
telecommunications, information processing, and in-
strumentation are being explored as vehicles for deliver-
ing measurement services more effectively.
3.1 Telemetrology
The Internet and new communication technologies
will influence metrology in this century, much as the
telephone, fax, and email did in the last. The early stages
of this influence can be seen in telemetrology projects
that began at NIST in 1998.
3.1.1 SIMnet
The Interamerican Metrology System (SIM) was es-
tablished in 1979 to assist the 13 Latin American mem-
ber countries set up and maintain NMIs. In the 1990s,
SIM was expanded to include most of the countries in
the Americas (now 32 members). A division of the
Organization of American States, SIM consists of five
geographical Metrology Regions: NORAMET (North
America), CAMET (Central America), CARIMET
(Caribbean), ANDIMET (Northern South America) and
SURAMET (Southern South America). One of the
main objectives of SIM is to harmonize the basic mea-
surement standards in each country in the hemisphere.
SIM provides a framework for international compari-
sons that support this objective.
SIM has sponsored international comparisons in
mass, pressure, volume, and electricity in the latter half
of the 1990s. These comparisons use traveling standards
that are calibrated at each of the participating laborato-
ries. The first electrical comparison was started with
five digital multimeters (DMMs), calibrated at NIST
(the pilot lab) in 1997, and sent to “pivot”labs in each
94

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
metrology region. Each of the pivot labs then circulated
the traveling standards to the NMIs within their region.
Communications between metrologists in SIM were
done by fax and email, which was just becoming avail-
able at most of the NMIs.
In the fall of 1998, a project began at NIST to create
a communications network between the NMIs in SIM.
Dubbed SIMnet [119, 120]; it is a network of computers
devoted to video and data conferencing through the In-
ternet to facilitate international comparisons, foster col-
laboration between metrologists, promote exchange of
information, standardize test procedures, and share soft-
ware and data.
Early experience with the Internet-based video con-
ferencing software demonstrated how sensitive perfor-
mance was to different hardware, software drivers, and
operating systems. As a result, dedicated SIMnet video
conferencing stations were designed. The stations in-
clude a desktop computer, digital camera, headset, and
software to compress and transmit audio and video. The
camera provides real-time video and high-quality still
images, allowing small hardware details and instrument
connections to be examined remotely via the Internet.
In addition to providing video conferencing capabil-
ity, the station has an important advantage in interna-
tional comparisons where computer-controlled instru-
ments are often used as traveling standards. Control
software is used to program instrument parameters such
as range, settling time, and averaging. However, it can be
difficult to verify that different control software is im-
plementing the agreed upon test procedure. With the
standard SIMnet station at each NMI, it is possible to run
the same test software at each lab.
Video conferencing tools available on the SIMnet
station include the following (see Fig. 24):
Chat is a text communication tool that can be viewed
and used by all participants in the meeting. When Inter-
net traffic reduces audio quality, a back-up form of
communication is needed. Chat is also useful in a multi-
point conference for questions or comments.
Whiteboard is an important communication and doc-
umentation accessory. Participants can paste graphs,
data, photographs, and pictures from other applications.
This preserves session information in an electronic note-
book available to all participants in the meeting. Unlike
Fig. 24. Typical video conference screen with video images of the participants on the right in the main panel, a shared spreadsheet in the upper
left, the chat window in the lower left, and the whiteboard (electronic notebook) highlighted in the center.
95

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
the real-time video, images pasted on the whiteboard
are the same quality for all participants.
Share is a useful tool in data conferencing. One par-
ticipant can share with the others the window of a cur-
rently running program, like a spreadsheet. All partici-
pants see the same window, as shown in Fig. 24.
Collaborate is an extension of sharing; the partici-
pants can not only observe the screen of the running
application but also control it.
In December 1998 SIMnet was unveiled at NIST.
Eleven other NMIs within SIM were presented with
SIMnet stations and instructed in their use. To join a
conference, participants log-on to the SIMnet server,
which is maintained at NIST. The main task of the server
is to provide audio and video to all participants in a
multipoint conference. This feature is not presently
available without a special server. The server sits outside
the NIST network firewall and thus can be accessed by
any NMI. Since its inauguration, SIMnet has been con-
tinuously tested, and in March 1999 it was used for
multipoint video conferencing during the final phase of
the SIM International Comparison of Electrical Units.
3.1.2 Internet-Assisted Measurement Assurance
Program
For many years, NIST has provided what is called a
Measurement Assurance Program service (MAP) for a
number of electrical quantities [81]. The MAP is de-
signed on the principle that it is more valuable to test the
customer’s calibration process, rather than the traveling
standard. In a typical MAP, a NIST-owned standard is
calibrated and shipped to the customer, where it is cali-
brated as an unknown. The standard and customer test
data are then returned to NIST where a follow-up cali-
bration and data analysis are performed. A calibration
report is issued for the customer’s test system rather than
just the traveling standard. Communication between
NIST and the customer during the test is by telephone or
email. Since the data are returned with the standard, if
something is done incorrectly, the usefulness of the cal-
ibration is diminished and the standard may have to be
returned to the customer for repeat measurements.
A program is evolving at NIST to allow a customer’s
process to be monitored by NIST staff during the test
[120, 121, 122]. It employs the Internet to improve com-
munications, so the customer can transfer test data,
download test procedures, and use NIST control soft-
ware for system evaluation. As in the SIMnet process, an
interactive Internet allows the customer’s“before”and
“after”data to be sent electronically to NIST where the
data analysis is performed. Based on experience with
this project and SIMnet, almost all measurement ser-
vices at NIST could utilize the capabilities of the Inter-
net at some time in the future. Of course, most traveling
electrical standards will not be replaced by code travel-
ing on a digital network (i.e., no unit other than the
second can be propagated digitally), but it appears that
the Internet will greatly enhance measurement quality
and efficiency.
3.2 The Absolute Ampere and the Quantum Age
Absolute experimental determinations of units are
now known as SI realizations, and the uncertainty of the
SI values of the electrical units are limited by the uncer-
tainty of their realizations in terms of the kilogram,
meter, and second. Results from the calculable capacitor
experiment (Sec. 2.3) and other determinations of the
fine-structure constant recently have been analytically
combined [123] to yield a value of RKwith a relative
standard uncertainty of 4 ⫻10⫺9. The Josephson con-
stant KJis based both on its direct measurement by
voltage balances and by combining RKwith a value of
the Planck constant, the latter obtained by realizing the
watt in a special way. This realization of the SI watt is
achieved by the moving-coil watt balance, which is a
modern version of the absolute ampere experiment.
3.2.1 The Moving-Coil Watt Balance
The NIST watt balance [82] has been designed to
measure the ratio of mechanical to electrical power,
linking the artifact kilogram, the meter, and the second
to the practical realizations of the ohm and the volt
derived from the QHE and the Josephson effect, respec-
tively. Based on the equations given earlier, the Joseph-
son voltages UJand quantized Hall resistances RH(i) are
linked to the atomic constants by
UJ=nf/KJ=nf
2e/h, (9)
RH(i)=RK/i=h/e2
i, (10)
where nand iare integers, eis the elementary charge, h
is the Planck constant, fis the frequency of the mi-
crowave radiation applied to a Josephson device, KJis
the Josephson constant, and RKis the von Klitzing con-
stant.
The experimental method of the moving-coil watt
balance, as first proposed by B. Kibble [124], consists of
two measurement modes. In the first mode, a voltage
reference Uis used to servo control the velocity (dz/dt)
of a coil (see Fig. 25) moving vertically in a radial
magnetic flux density. In the second mode, a current I
96

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 25. Schematic of the 1990s NIST watt balance experiment. The wheel, both magnets, and the fixed induction coil are
rigidly connected. A cryostat is between the superconducting magnet and the induction coils.
passing through the same coil, now held stationary in the
same magnetic flux density, is used to balance the force
Fz=mg , where mis the mass of a standard mass and g
is the local acceleration of gravity. The equation,
Fz/I=mg /I=⫺d
⌽
/dz=U
dz/dt, (11)
where d
⌽
/dzis the vertical magnetic flux density gradi-
ent in the coil, relates the two modes. This can be
rewritten as
UI =mg (dz/d t)=F(dz/dt), (12)
which equates the electric power (measured in our labo-
ratory units) and the mechanical power (in SI units).
97

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
The utilization of two separate modes of measurement
is the reason that this equation can be realized with a
small uncertainty. In the “velocity mode” no current
flows in the moving coil and no power is dissipated. In
the “balance mode”, the power dissipation from friction
is negligible for the minimal motion of the balance and
coil. Thus this experiment uses Eq. (12) to equate two
types of virtual power, one due to gravity and one due
to electrical forces.
The formal definitions for the practical units (1990
representations) of voltage KJ⫺90 and resistance RK⫺90 are
given in Eq. (2) and Eq. (5). Rewriting Eq. (12) to
explicitly indicate the units used in the experiment, we
obtain
{UI }90 W90 ={mg dz/dt}SI Wor
W90
W={mg dz/dt}SI
{UI }90
, (13)
where W90 is the conventional unit of power based on the
Josephson and quantum Hall effects and the conven-
tional values of KJ⫺90 and RK⫺90, W is the SI watt, {}SI
and {}90 mean the numerical value of the quantity in
curly brackets when expressed in SI units, or 1990 prac-
tical units. Eq. (13) shows that the ratio W90/W is the
direct result from observations made in the two modes
of the watt experiment.
We next show that the value of certain fundamental
constants are also obtained from the moving-coil watt
balance experiment. Using Eqs. (9) and (10), we find
KJ2RK=4/h. From V/KJ=V90/KJ⫺90,RK⍀=RK⫺90
⍀
90,
and W90 =(V90)2/
⍀
90 (where V90 and
⍀
90 are the 1990
units of voltage and resistance) we then obtain values for
hand KJ,
h=4(W90/W)
K2
J⫺90 RK⫺90
, (14)
KJ= (4/hRK)1/2 =KJ⫺90[(RK⫺90/RK)W/W90]1/2. (15)
Watt-balance measurements of hand KJdo not depend
on the values chosen for KJ⫺90 and RK⫺90 as long as the
JVS and QHE are used to measure Uand Ibased on Eq.
(13). However, KJ⫺90 was chosen using the measure-
ments available in 1990 to make W90/W = 1 and any
measured deviation means the conventional values KJ⫺90
and RK⫺90 would need to be adjusted to preserve this
equality.
The first results from the NIST watt experiment,
sometimes called an ampere experiment, were published
in 1989 [30], giving a relative standard uncertainty for
KJof 6.7 ⫻10⫺7. That experiment was a prototype for
the next version in which the magnetic field was in-
creased a factor of fifty using a superconducting mag-
net, resulting in similar increases in the force and
voltage. During the next decade many improvements
were made [125, 126]. In 1998 the latest results were
published [82] by E. R. Williams, R. L. Steiner, D. B.
Newell, and P. T. Olsen. That work reports that KJ=
483 597.892 GHz/V with a relative standard uncertainty
of 4.4 ⫻10⫺8using a NIST calculable capacitor mea-
surement of RK. This experiment provided the most ac-
curate measurement of this quantity to date, and is de-
scribed in the next section.
3.2.2 A Description of the 1990s NIST Watt
Experiment
Figure 25 shows the configuration of the NIST Watt
experiment. The axial force on a loop of wire of radius
ain a purely radial field, B=[Ba(z)/r]r^, where Ba(z)is
nearly constant with zand time, is independent of the
wire shape. A superconducting magnet with 200 000
turns, consisting of two solenoid sections wound in op-
position, produces a 0.1 T radial field outside the mag-
net cryostat. Two induction coils, each with 2355 turns,
are located in the radial field. The lower induction coil
is fixed to the support structure and acts as a position
reference. The upper induction coil is suspended from a
balance made from a pivoting wheel located above the
cryostat. This allows the coil to move strictly vertically
for 100 mm as the wheel rotates through an angle of
⫾10⬚. Sensors monitor the five rotational, tilting, and
translational modes of motion for the induction coil,
other than vertical. By using data on the coil motion
along with some mutual inductance techniques, one is
able to align the experiment and to estimate the align-
ment errors [125]. Also, the unwanted motion can be
actively damped using data from these sensors.
One group of recent measurements recorded 989 val-
ues of the SI watt over a 4 month period. The total
uncertainty is dominated by Type B uncertainty compo-
nents, that is, components that have to be evaluated by
means other than statistical analysis of repeated mea-
surements. Of the possible Type B error sources [126]
that contribute to the uncertainty, the three largest com-
ponents arise from the following: (1) the index of refrac-
tion of air; (2) the present alignment procedures; and (3)
residual knife-edge hysteresis effects during force mea-
surements. Using the data discussed above Williams
et al. obtained a relative standard uncertainty of
0.087 ␮W/W. The final result is (W90/W ⫺1) =
(+0.8 ⫾8.7) ⫻10⫺8,h= 6.626 068 91 (58) ⫻10⫺34 Js,
and (KJ⫺90/KJ⫺1)=(⫺1.6 ⫾4.4) ⫻10⫺8.
98

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
By connecting the macroscopic unit of mass (the kilo-
gram) to quantum standards based on the Josephson and
quantum Hall effects, this result provides a significant
improvement in the Josephson constant as well as many
other constants. Figure 26 compares recent measure-
ments of the Plank constant h, which can be derived
directly from this work with a relative standard uncer-
tainty of 8.7 ⫻10⫺8.
Fig. 26. Comparison of recent electrical measurements of the Planck
constant h. NIM: National Institute of Metrology (People’s Republic
of China); NPL: National Physical Laboratory (UK); PTB: Physikalis-
che-Technische Bundesanstalt, (Germany); CSIRO/NML: National
Measurement Laboratory (Australia); CODATA: Committee on Data
for Science and Technology of the International Council of Scientific
Unions, Task Group on Fundamental Constants. The CODATA value
of his a least-squares adjusted value based on the other measurements
shown here.
3.2.3 New Construction and Monitoring of the
Kilogram
The NIST watt experiment is being completely rebuilt
to achieve an improvement by a factor of ten, to less than
10 nW/W relative standard uncertainty. At that level of
measurement uncertainty, the watt-balance experiment
becomes a very good means of monitoring the mass
artifact that is used in the weighings. The present defini-
tion of the unit of mass in the SI is based on the Interna-
tional Prototype of the Kilogram, which is a cylinder of
platinum-iridium housed at the BIPM in France. The
Prototype and a set of duplicate standards of mass accu-
mulate contaminants on their surfaces, and must be
cleaned to achieve fractional changes over the long term
of less than 10⫺8per year. Since the kilogram is the last
artifact SI base unit defined in terms of a material
artifact, a quantum standard of mass founded on electri-
cal measurements would complete the modern trend of
removing all artifacts from the definitions of SI units.
The largest uncertainties in the 1990s NIST watt ex-
periment arose from operating in air, which required
that the changing air buoyancy and refractive index be
calculated from many readings of pressure, tempera-
ture, and humidity sensors. Almost every part of the
balance assembly is being rebuilt to operate inside a
specially constructed vacuum system consisting of two
chambers, schematically represented in Figure 27. The
upper chamber houses the balance section. A toroid-
shaped chamber houses the inductive coils, located 3 m
below and centered about the liquid helium cryostat
containing the superconducting magnet.
One of the powerful aspects of the relationships in
fundamental constants is that we can often derive one
from a combination of others. The “electronic kilogram”
experiment is designed to achieve sufficient accuracy to
allow redefinition of the artifact kilogram in terms of a
fundamental constant such as the Planck constant hor
the mass of an atom such as 12C [127, 128]. This redefi-
nition of mass is likely to be the last major change in the
SI for many decades. Of interest here is the calculation
of the mass of an atom from measurements of the mov-
ing coil watt balance. Using the theory for the Rydberg
constant R⬁, the following equation relates the SI mass
of 12C to the mass used to measure the ratio W90/W in
this experiment. From Eqs. (14) and (16),
R⬁=m(e)c
␣
2/2h, (16)
m(12C) = 冉W90
W冊冋8冉m(e)
m(12C)冊⫺1R⬁
c
␣
2K2
J⫺90 RK⫺90册. (17)
Here cis the speed of light, (m(e)/m(12C)) is the elec-
tron to 12C mass ratio, and
␣
is the fine-structure con-
stant. The relative combined uncertainty in the group of
constants inside the square bracket in Eq. (17) is below
8⫻10⫺9. At present, this watt balance experiment is the
most accurate determination of m(12C) and any im-
provement will provide a corresponding improvement in
m(12C). Thus, its measurement connects the macro-
scopic kilogram to the atomic mass scale.
Scientists of the Electricity Division wish to further
improve the accuracy of the watt balance. If it proves
possible to connect the mass of the artifact kilogram to
m(12C) with accuracy equal or better than the accepted
long-term stability of the artifact, then it would be time
to replace the current definition for the kilogram with
99

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 27. Schematic representation of the electronic kilogram apparatus. The vacuum chamber and support tripod
are shown in cut-away view.
one based on defining an atomic mass. By defining
mass in a way similar to the way time and length are now
defined, relative to a Cesium hyperfine frequency
(atomic clock) and the speed of light respectively, this
will eliminate the last artifact standard in the SI.
4. Conclusion
Contributions from NIST have helped to provide ab-
solute determinations of the values of the ampere and
the ohm, and to develop quantum standards that are
universal, allowing electrical quantities to be determined
in units that do not change with time and that are repro-
ducible in any laboratory. The broader achievement has
been success in providing electrical standards and mea-
surements of the finest quality. The authors would like
to recognize the contributions of many not already men-
tioned by name, including F. B. Silsbee, F. K. Harris, B.
N. Taylor, N. B. Belecki, W. D. Phillips, and all those
who have built and maintained experiments, standards,
and services at NIST.
5. References
[1] J. L. Thomas, Precision resistors and their measurement, NBS
Circ. 470 (1948).
[2] F. B. Silsbee, Establishment and maintenance of the electrical
units, NBS Circ. 475 (1949).
[3] F. K. Harris, Electrical units, in Proc. 19th An. Instrum. Soc.
Amer. Conf. (Paper No. 12.1-1-64) (1964).
[4] R. C. Cochrane, Measures for Progress—A History of the Na-
tional Bureau of Standards, U.S. Dept. of Commerce, Washing-
ton, DC (1966).
[5] E. B. Rosa, Report to the International Committee on Electrical
Units and Standards, NBS Misc. Publ. M16 (1912).
[6] H. L. Curtis and R. W. Curtis, An absolute determination of the
ampere, Bur. Stand. J. Res. 12, 665-734 (1934).
[7] H. L. Curtis, C. Moon, and C. M. Sparks, An absolute determi-
nation of the ohm, J. Res. Natl. Bur. Stand. (U.S.) 16, 1-82
(1936).
100

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
[8] H. L. Cur tis, C. Moon, and C. M. Sparks, An absolute determi-
nation of the ohm using an improved self inductor, J. Res. Natl.
Bur. Stand. (U.S.) 21, 375-423 (1938).
[9] J. L. Thomas, A new design of precision resistance standard, Bur.
Stand. J. Res. 5, 295-304 (1930).
[10] J. L. Thomas, Stability of double-walled manganin resistors, J.
Res. Natl. Bur. Stand. (U.S.) 36, 107-110 (1946).
[11] H. L. Cur tis, Review of recent absolute determinations of the
ohm and ampere, J. Res. Natl. Bur. Stand. (U.S.) 33, 235-254
(1944).
[12] J. L. Thomas, An absolute measurement of resistance by the
Wenner method, J. Res. Natl. Bur. Stand. (U.S.) 43, 291-353
(1949).
[13] R. D. Cutkosky, Evaluation of the NBS unit of resistance based
on a calculable capacitor, J. Res. Natl. Bur. Stand. (U.S.) 65A,
147-158 (1961).
[14] R. D. Cutkosky, Evaluation of the NBS unit of resistance based
on a calculable capacitor, Precision measurement and calibra-
tion, electricity—low frequency, NBS Spec. Publ. 300, pt. 3,
(70-4) (1968).
[15] R. L. Driscoll, Measurement of current with a Pellat-type elec-
trodynamometer, J. Res. Natl. Bur. Stand. (U.S.) 60, 287-296
(1958).
[16] R. L. Driscoll and R. D. Cutkosky, Measurement of current with
the National Bureau of Standards current balance, J. Res. Natl.
Bur. Stand. (U.S.) 60, 297-305 (1958).
[17] N. B. Belecki, R. F. Dziuba, B. F. Field, and B. N. Taylor,
Guidelines for implementing the new representations of the volt
and the ohm effective January 1, 1990, NIST Tech. Note 1263
(1989).
[18] R. F. Dziuba, The NBS ohm: past-present-future, in Proc. Meas.
Sciences Conf. (Paper No. VI-A, 15) (1987).
[19] S. Lindeck, British Association Report of Standard Committee
for 1892, Appendix IV, (1892).
[20] E. B. Rosa, A new form of standard resistance, Bull. Bur. Stand.
12, 375 (1908).
[21] L. Clark, On a voltaic standard of electromotive force, Proc. R.
Soc. London 20, 444 (1872).
[22] W. J. Hamer, Standard cells—their construction, maintenance,
and characteristics, NBS Monogr. 84 (1965).
[23] R. L. Driscoll and P. T. Olsen, Application of nuclear resonance
to the monitoring of electrical standards, in Precision Measure-
ments and Fundamental Constants, D. N. Langenberg and B. N.
Taylor, eds., NBS Spec. Publ. 343 (1971) pp. 117-121.
[24] W. G. Eicke, Commentaires sur l’utilisation des diodes de Zener
comme etalons de tension, Comite´Consultatif d’E
´lectricite´, 11e
Session, 1874-1877, BIPM, Paris, France (1963).
[25] E. R. Williams and P. T. Olsen, New measurement of the proton
gyromagnetic ratio and a derived value of the fine-structure
constant, Phys. Rev. Lett. 42, 1575-1579 (1979).
[26] E. R. Williams, G. R. Jones Jr., S. Ye, R. Liu, H. Sasaki, P. T.
Olsen, W. D. Phillips, and H. P. Layer, A low field determination
of the proton gyromagnetic ratio in water, IEEE Trans. Instrum.
Meas. 38(2), 233-237 (1989).
[27] R. L. Steiner and B. F. Field, Josephson array voltage calibration
system—operational use and verification, IEEE Trans. Instrum.
Meas. 38(2), 296-301 (1989).
[28] J. Q. Shields, R. F. Dziuba, and H. P. Layer, New realization of
the ohm and farad using the NBS calculable capacitor, IEEE
Trans. Instrum. Meas. 38(2), 249-251 (1989).
[29] M. E. Cage, R. F. Dziuba, C. T. Vandegrift, and D. Y. Yu,
Determination of the time-dependence of ⍀NBS using the quan-
tized Hall resistance, IEEE Trans. Instrum. Meas. 38(2), 263-269
(1989).
[30] P. T. Olsen, R. E. Elmquist, W. D. Phillips, E. R. Williams, G. R.
Jones, and V. E. Bower, A measurement of the NBS electrical
watt in SI units, IEEE Trans. Instrum. Meas. 38(2), 238-244
(1989).
[31] M. E. Cage, R. F. Dziuba, R. E. Elmquist, B. F. Field, G. R.
Jones, P. T. Olsen, W. D. Phillips, J. Q. Shields, R. L. Steiner, B.
N. Taylor, and E. R. Williams, NBS determination of the fine-
structure constant, and of the quantized Hall resistance and
Josephson frequency-to-voltage quotient in SI units, IEEE Trans.
Instrum. Meas. 38(2), 284-289 (1989).
[32] B. D. Josephson, Possible new effects in superconductive tunnel-
ing, Phys. Lett. 1, 251-253 (1962).
[33] K. von Klitzing, G. Dorda, and M. Pepper, New method for high
accuracy determination of the fine-structure constant based on
quantized Hall resistance, Phys. Rev. Lett. 45, 494-497 (1980).
[34] B. D. Josephson, Supercurrents through barriers, Adv. Phys. 14,
419-451 (1965).
[35] P. W. Anderson and J. M. Rowell, Probable observation of the
Josephson superconducting tunneling effect, Phys. Rev. Lett. 10,
230-232 (1963).
[36] S. Shapiro, Josephson currents in superconducting tunneling: the
effects of microwaves and other observations, Phys. Rev. Lett.
11, 80-82 (1963).
[37] J. Clark, Experimental comparison of the Josephson voltage-fre-
quency relation in different superconductors, Phys. Rev. Lett. 21,
1566-1569 (1968).
[38] W. H. Parker, D. N. Langenberg, A. Denenstein, and B. N.
Taylor, Determination of e/h, using macroscopic quantum phase
coherence in superconductors: 1. Experiment, Phys. Rev. 177,
639-664 (1969).
[39] T. F. Finnegan, A. Denenstein, and D. N. Langenberg, Ac-
Josephson-effect determination of e/h: a standard of electro-
chemical potential based on macroscopic quantum phase oher-
ence in superconductors, Phys. Rev. B: Condens. Matter 4,
1487-1522 (1971).
[40] F. Bloch, Josephson effect in a superconducting ring, Phys. Rev.
B: Condens. Matter 2, 109-121 (1970).
[41] T. A. Fulton, Implications of solid-state corrections to the
Josephson voltage-frequency relation, Phys. Rev. B: Condens.
Matter 7, 981-982 (1973).
[42] T. D. Bracken and W. O. Hamilton, Comparison of microwave-
induced constant-voltage steps in Pb and Sn Josephson junctions,
Phys. Rev. B: Condens. Matter 6, 2603-2609 (1972).
[43] J. C. Macfarlane, Comparison between constant-potential steps
of two point-contact Josephson junctions, Appl. Phys. Lett. 22,
549-550 (1973).
[44] B. N. Taylor, W. H. Parker, D. N. Langenberg, and A. Denen-
stein, On the use of the ac Josephson effect to maintain standards
of electromotive force, Metrologia 3(4), 89-98 (1967).
[45] F. K. Harris, H. A. Fowler, and P. T. Olsen, Accurate Hamon-pair
potentiometer for Josephson frequency-to-voltage measure-
ments, Metrologia 6, 134-142 (1970).
[46] B. F. Field, T. F. Finnegan, and J. Toots, Volt maintenance at NBS
via 2e/h: a new definition of the NBS volt, Metrologia 9, 155-
166 (1973).
[47] W. G. Eicke and B. N. Taylor, Summary of international com-
parisons of as-maintained units of voltage and values of 2e/h,
IEEE Trans. Instrum. Meas. 21, 316-319 (1972).
[48] B. N. Taylor, History of the present value of 2e/hcommonly used
for defining national units of voltage and possible changes in
national units of voltage and resistance, IEEE Trans. Instrum.
Meas. 36(2), 659-664 (1987).
[49] M. T. Levinsen, R. Y. Chiao, M. J. Feldman, and B. A. Tucker,
An inverse ac Josephson effect voltage standard, Appl. Phys.
Lett. 31(11), 776-778 (1977).
101

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
[50] R. L. Kautz and F. L. Lloyd, Precision of series-array Josephson
voltage standards, Appl. Phys. Lett. 51(24), 2043-2045 (1987).
[51] R. L. Kautz, Noise, chaos, and the Josephson voltage standard,
Reports on Progress in Physics 59(8), 935-992 (1996).
[52] J.-S. Tsai, A. K. Jain, and J. E. Lukens, High-precision test of the
universality of the Josephson voltage-frequency relation, Phys.
Rev. Lett. 51, 316-319 (1983).
[53] C. A. Hamilton, R. L. Kautz, R. L. Steiner, and F. L. Lloyd, A
practical Josephson voltage standard at 1 V, IEEE Electron
Device Lett. 6(12), 623-625 (1985).
[54] J. Niemeyer, L. Grimm, W. Meier, J. H. Hinken, and E. Vollmer,
Stable Josephson reference voltages between 0.1 and 1.3 V for
high-precision voltage standards, Appl. Phys. Lett. 47(11), 1222-
1223 (1985).
[55] J. Niemeyer and L. Grimm, Present status of Josephson voltage
met rology, Metrologia 25(3), 135-140 (1988).
[56] C. A. Hamilton, F. L. Lloyd, K. Chieh, and W. C. Goeke, A 10-V
Josephson voltage standard, IEEE Trans. Instrum. Meas. 38(2),
314-316 (1989).
[57] B. N. Taylor and T. J. Witt, New international electrical reference
standards based on the Josephson and quantum Hall effects,
Metr ologia 26(1), 47-62 (1989).
[58] R. E. Prange and S. M. Girvin, The Quantum Hall Effect,
Springer-Verlag, New York (1987).
[59] G. Landwehr, The discovery of the quantum Hall effect,
Metr ologia 22, 118-127 (1986).
[60] R. B. Laughlin, Quantized Hall conductivity in two dimensions,
Phys. Rev. B: Condens. Matter 23, 5632-5633 (1981).
[61] R. J. Wagner, C. F. Lavine, M. E. Cage, R. F. Dziuba, and B. F.
Field, Measurements of the quantized Hall steps in Si at the ppm
level, Surf. Sci. 113, 10-15 (1982).
[62] D. C. Tsui, A. C. Gossard, B. F. Field, M. E. Cage, and R. F.
Dziuba, Determination of the fine-structure constant using
GaAs/AlGaAs heterostructures, Phys. Rev. Lett. 48, 3-6 (1982).
[63] G. M. Reedtz and M. E. Cage, An automated potentiometric
system for precision measurement of the quantized Hall resis-
tance, J. Res. Natl. Bur. Stand. (U.S.) 92(5), 303-310 (1987).
[64] A. Har tland, R. G. Jones, B. P. Kibble, and D. J. Legg, The
relationship between the SI ohm, the ohm at NPL, and the quan-
tized Hall resistance, IEEE Trans. Instrum. Meas. 36(2), 208-213
(1987).
[65] G. W. Small, B. W. Ricketts, and P. C. Coogan, A reevaluation of
the NML absolute ohm and quantized Hall resistance determina-
tions, IEEE Trans. Instrum. Meas. 38(2), 245-248 (1989).
[66] F. Delahaye, Technical guidelines for reliable measurements of
the quantized Hall resistance, Metrologia 26(1), 63-68 (1989).
[67] M. E. Cage, B. F. Field, R. F. Dziuba, S. M. Girvin, A. C.
Gossard, and D. C. Tsui, Temperature-dependence of the quan-
tum Hall resistance, Phys. Rev. B: Condens. Matter 30(4), 2286-
2288 (1984).
[68] M. E. Cage, R. F. Dziuba, B. F. Field, E. R. Williams, S. M.
Girvin, A. C. Gossard, D. C. Tsui, and R. J. Wagner, Dissipation
and dynamic non-linear behavior in the quantum Hall regime,
Phys. Rev. Lett. 51(15), 1374-1377 (1983).
[69] K. C. Lee, Degradation of GaAs/AlGaAs quantized Hall resis-
tors with alloyed AuGe/Ni contacts, J. Res. Natl. Inst. Stand.
Technol. 103(2), 177-200 (1998).
[70] A. Har tland, K. Jones, J. M. Williams, B. L. Gallagher, and T.
Galloway, Direct comparison of the quantized Hall resistance in
gallium-arsenide and silicon, Phys. Rev. Lett. 66(8), 969-973
(1991).
[71] B. Jeckelmann, A. D. Inglis, and B. Jeanneret, Material, device,
and step independence of the quantized Hall resistance, IEEE
Trans. Instrum. Meas. 44(2), 269-272 (1995).
[72] A. Har tland, The quantum Hall effect and resistance standards,
Metr ologia 29(2), 175-190 (1992).
[73] F. Piquemal, L’effet Hall quantique en metrologie, Bulletin du
Bureau National de Metrologie 1999-2 (116), 5-57 (1999).
[74] M. W. Keller, J. M. Martinis, N. N. Zimmerman, and A. H.
Steinbach, Accuracy of electron counting using a 7-junction
electron pump, Appl. Phys. Lett. 69(12), 1804-1806 (1996).
[75] P. Delsing and T. Claeson, Single electron turnstile and pump
devices using long arrays of small tunnel-junctions, Phys. Scr.
T42, 177-181 (1992).
[76] M. W. Keller, A. L. Eichenberger, J. M. Martinis, and N. M.
Zimmerman, A capacitance standard based on counting elec-
trons, Science 285(5434), 1706-1709 (1999).
[77] Y. H. Tang, R. Steiner, and J. Sims, Comparison of two Joseph-
son array voltage standard systems using a set of Zener refer-
ences, IEEE Trans. Instrum. Meas. 48(2), 262-265 (1999).
[78] R. L. Steiner, A. F. Clark, C. Kiser, T. J. Witt, and D. Reymann,
Comparison of two Josephson array voltage standard systems
using a set of Zener references, IEEE Trans. Instrum. Meas.
48(2), 262-265 (1993).
[79] T. J. Witt, D. Reymann, Y. H. Tang, and C. A. Hamilton, Bilat-
eral Comparison of 10 V Standards Between the NIST, Gaithers-
burg, the NIST, Boulder, and the BIPM, October 1998 to Janu-
ary, 1999, Rapport BIPM-99/07, BIPM, Paris, France (1999).
[80] R. L. Steiner and S. Starley, MAP voltage transfer between 10 V
Josephson array systems, in Proc. NCSL Workshop and Symp.
NCSL’91, Natl. Conf. Stand. Lab., Boulder, CO (1991) pp. 205-
209.
[81] J. Marshall, NIST calibration services users guide, Natl. Inst.
Stand. Technol. Spec. Publ. 250 (1998).
[82] E. R. Williams, R. L. Steiner, D. B. Newell, and P. T. Olsen,
Accurate measurement of the Planck constant, Phys. Rev. Lett.
81(12), 2404-2407 (1998).
[83] C. J. Burroughs, S. P. Benz, C. A. Hamilton, and T. E. Harvey,
Programmable1Vdcvoltage standard, IEEE Trans. Instrum.
Meas. 48(2), 279-281 (1999).
[84] F. L. Hermach, Thermal converters as ac-dc transfer standards
for current and voltage measurements at audio frequencies, J.
Res. Natl. Bur. Stand. (U.S.) 48, 121-138 (1952).
[85] F. L. Hermach and E. S. Williams, Thermal voltage converters
for accurate voltage measurements to 30 megacycles per second,
Comm. Elec. 79, pt.1, 200-206 (1960).
[86] J. R. Kinard, T. E. Lipe, C. B. Childers, and S. Avramov-Za-
murovic, Comparison of high voltage thermal converters scaling
to a binary inductive voltage divider, in Digest Conf. Prec. Elec-
trom. Meas. (IEEE Cat. No. 98CH36245), 381-382 (1998).
[87] J. R. Kinard, D. X. Huang, and D. B. Novotny, Performance of
multilayer thin-film multijunction thermal converters, IEEE
Trans. Instrum. Meas. 44(2), 383-386 (1995).
[88] C. D. Reintsema, E. N. Grossman, J. A. Koch, J. R. Kinard, and
T. E. Lipe, Ac-dc transfer at cryogenic temperatures using a
superconducting resistive transition-edge temperature sensor, in
Proc. Instrum. Meas. Technol. Conf. IMTC’97 (1), 726-730
(1997).
[89] C. D. Reintsema, J. R. Kinard, T. E. Lipe, J. A. Koch, and E. N.
Grossman, Thermal transfer measurements at microwatt power
levels, in Digest Conf. Prec. Electrom. Meas. (IEEE Cat. No.
98CH36254), 171-172 (1998).
[90] A. M. Thompson and D. G. Lampard, A new theorem in electro-
statics and its application to calculable standards of capacitance,
Nature 177, 888 (1956).
[91] W. K. Clothier, A calculable standard of capacitance, Metrologia
1, 36-55 (1965).
102

Volume 106, Number 1, January–February 2001
Journal of Research of the National Institute of Standards and Technology
[92] R. D. Cutkosky, New NBS measurements of the absolute farad
and ohm, IEEE Trans. Instrum. Meas. 23, 305-309 (1974).
[93] R. D. Cutkosky and L. H. Lee, Improved ten-picofarad fused
silica dielectric capacitor, J. Res. Natl. Bur. Stand. (U.S.) 69C,
173-179 (1965).
[94] A. M. Jeffery, R. E. Elmquist, L. H. Lee, J. Q. Shields, and R.
F. Dziuba, NIST comparison of the quantized Hall resistance
and the realization of the SI ohm through the calculable capac-
itor, IEEE Trans. Instrum. Meas. 46(2), 264-268 (1997).
[95] A. Jeffery, R. E. Elmquist, J. Q. Shields, L. H. Lee, M. E. Cage,
S. H. Shields, and R. F. Dziuba, Determination of the von
Klitzing constant and the fine-structure constant through a com-
parison of the quantized Hall resistance and the ohm derived
from the NIST calculable capacitor, Metrologia 35(2), 83-96
(1998).
[96] A. Jeffery, L. H. Lee, and J. Q. Shields, Model tests to investi-
gate the effects of geometrical imperfections on the NIST cal-
culable capacitor, IEEE Trans. Instrum. Meas. 48, 356-359
(1999).
[97] E. R. Cohen and B. N. Taylor, The 1986 adjustment of the
fundamental physical constants, Rev. Mod. Phys. 59(4), 1121-
1148 (1987).
[98] B. N. Taylor and E. R. Cohen, Recommended values of the
fundamental physical constants—a status report, J. Res. Natl.
Inst. Stand. Technol. 95(5), 497-523 (1990).
[99] V. W. Hughes and T. Kinoshita, Anomalous g values of the
electron and muon, Rev. Mod. Phys. 71, S133-S139 (1999).
[100] T. L. Zapf, Calibration of inductance standards in the Maxwell-
Wien bridge circuit, J. Res. Nat. Bur. Stand. (U.S.) 65C, 183-
188 (1961).
[101] J. Melcher, P. Warnecke, and R. Hanke, Comparison of preci-
sion ac and dc measurements with the quantized Hall resistance,
IEEE Trans. Instrum. Meas. 42(2), 292-294 (1993).
[102] F. Delahaye, Accurate ac measurements of the quantized Hall
resistance from 1-Hz to 1,6 kHz, Metrologia 31(5), 367-373
(1995).
[103] A. Har tland, B. P. Kibble, P. J. Rodgers, and J. Bohacek, Ac
measurements of the quantized Hall resistance, IEEE Trans.
Instrum. Meas. 44(2), 245-248 (1995).
[104] B. M. Wood, A. D. Inglis, and M. Cote, Evaluation of the ac
quantized Hall resistance, IEEE Trans. Instrum. Meas. 46(2),
269-272 (1997).
[105] J. Bohacek, P. Svoboda, and P. Vasek, Ac QHE-based calibra-
tion of resistance standards, IEEE Trans. Instrum. Meas. 46(2),
273-275 (1997).
[106] M. E. Cage, R. F. Dziuba, and B. F. Field, A test of the quantum
Hall-effect as a resistance standard, IEEE Trans. Instrum. Meas.
34(2), 301-303 (1985).
[107] R. F. Dziuba and R. E. Elmquist, Improvements in resistance
scaling at NIST using cryogenic current comparators, IEEE
Trans. Instrum. Meas. 42(2), 126-130 (1993).
[108] D. B. Sullivan and R. F. Dziuba, Low temperature direct current
comparators, Rev. Sci. Instrum. 45, 517-519 (1974).
[109] R. E. Elmquist and R. F. Dziuba, Isolated ramping current
sources for a cryogenic current comparator bridge, Rev. Sci.
Instrum. 62(10), 2457-2460 (1991).
[110] F. Delahaye, T. J. Witt, R. E. Elmquist, and R. F. Dziuba,
Comparison of quantum Hall effect resistance standards of the
NIST and the BIPM, Metrologia, to be published.
[111] R. F. Dziuba, D. G. Jarrett, L. L. Scott, and A. J. Secula,
Fabrication of high-value standard resistors, IEEE Trans. In-
strum. Meas. 48(2), 333-337 (1999).
[112] F. Wenner, Methods, apparatus, and procedures for the com-
parison of precision standard resistors, J. Res. Natl. Bur. Stand.
(U.S.) 25, 229-293 (1940).
[113] S. K. Basu and N. L. Kusters, Comparison of standard resistors
by the dc current comparator, IEEE Trans. Instrum. Meas.
14(3), 149-156 (1965).
[114] B. V. Hamon, A 1-100 ⍀build-up resistor for the calibration of
standard resistors, J. Sci. Instrum. 31, 450-453 (1954).
[115] R. F. Dziuba, P. A. Boynton, R. E. Elmquist, D. G. Jarrett, T. M.
Moore, and J. D. Neal, NIST measurement service for standard
resistors, Natl. Inst. Stand. Technol. Tech. Note 1298 (1992).
[116] K. R. Baker and R. F. Dziuba, Automated NBS 1 ⍀measure-
ment system, IEEE Trans. Instrum. Meas. 32(1), 154-158
(1983).
[117] R. F. Dziuba and L. L. Kile, An automated guarded bridge
system for the comparison of 10 k⍀standard resistors, IEEE
Trans. Instrum. Meas. 48(3), 673-677 (1999).
[118] D. G. Jarrett, Automated guarded bridge for calibration of mul-
timegohm standard resistors from 10 M⍀to1T⍀, IEEE Trans.
Instrum. Meas. 46(2), 325-329 (1997).
[119] P. Filipski and N. M. Oldham, SIMnet—a collaborative tool for
metrology in the Americas, in Conf. Record 16th IEEE In-
strum. Meas. Technol. Conf. (IEEE Cat. No. 99CH36309),
Venice, Italy (1999).
[120] N. M. Oldham and M. Parker, Internet-based test service for
multifunction calibrators, in Conf. Record 16th IEEE Instrum.
Meas. Technol. Conf. (IEEE Cat. No. 99CH36309), Venice,
Italy (1999).
[121] N. M. Oldham, M. Parker, A. Young, and A. Smith, A high-ac-
curacy, 10 Hz-1 MHz automatic ac voltage calibration system,
IEEE Trans. Instrum. Meas. 36, 883-887 (1987).
[122] N. M. Oldham and M. Parker, NIST Multifunction Calibration
System, Natl. Inst. Stand. Technol. Spec. Publ. 250-46 (1998).
[123] P. J. Mohr and B. N. Taylor, CODATA recommended values of
the fundamental constants: 1998, J. Phys. Chem. Ref. Data 28
(6), 1713-1852 (1999), Rev. Mod. Phys. 72(2), 351-495 (2000).
[124] B. P. Kibble, “A measurement of the gyromagnetic moment of
the proton by using the strong field method,”Atomic Masses
and Fundamental constants, Vol. 5, J. H. Sanders and A. H.
Wapstra, eds., Plenum, New York (1976) pp. 545-551.
[125] A. D. Gillespie, K. Fujii, D. B. Newell, P. T. Olsen, A. Picard,
R. L. Steiner, G. N. Stenbakken, and E. R. Williams, Alignment
uncertainties of the NIST watt experiment, IEEE Trans. In-
strum. Meas. 46(2), 605-608 (1997).
[126] R. L. Steiner, D. B. Newell, and E. R. Williams, A result from
the NIST watt balance and an analysis of uncertainties, IEEE
Trans. Instrum. Meas. 48(2), 205-208 (1999).
[127] B. N. Taylor, The possible role of the fundamental constants in
replacing the kilogram, IEEE Trans. Instrum. Meas. 40(2), 86-
91 (1991).
[128] B. N. Taylor and P. J. Mohr, On the redefinition of the kilogram,
Metr ologia 36, 63-64 (1999).
About the authors: Randolph E. Elmquist, Marvin E.
Cage, Yi-hua Tang, Anne-Marie Jeffery, Joseph R. Ki-
nard, Jr., Ronald F. Dziuba, Nile M. Oldham, and Ed-
win R. Williams are physicists or electrical engineers in
the Electricity Division of the Electronics and Electrical
Engineering Laboratory of NIST. They are actively in-
volved in advancing fundamental electrical measure-
ments and developing standards and methods for the
maintenance of electrical units.
103