Helping to retire “Le Grand K”

“Le Grand K” has been the stand-setting definition of the kilogram for the last 127 years.

However, it has only been taken out from under its multiple protective glass bell jars three times for fear of damaging it. Stephan Schlamminger, National Institute of Standards and Technology (NIST), is working to define the kilogram using a value that can’t be changed by a fingerprint or a falling hair.

The study published in the journal Review of Scientific Instruments reveals that the machine that hopes to replace “Le Grand K” has taken its first full measurement. We spoke to Stephan Schlamminger about the results.

ResearchGate: How did you find yourself in this field of research?

Schlamminger: I worked on my PhD at the University of Zurich, Switzerland measuring the gravitational constant. This was my first foray into metrology and I fell in love with the field. There is only one thing that is better than a measurement: two measurements. The art of measurements is something fascinating and interesting.

RG: What is the “Le Grand K”? Why do we need to find something to replace it?

Schlamminger: The Grand K is the artifact that defines the unit of mass, the kilogram. Artifact based metrology is problematic, because one has to use the artifact in order to make the measurement, but, using the artifact can change or damage it. So, there is a tension between using and not-using and it is difficult to find the right balance. For the most part the Grand K has been kept in a vault. It has only been taken out three times. Clearly, there needs to be a more practical definition of the unit of mass.

A replica of the prototype kilogram on display at Cité des Sciences et de l'Industrie, featuring the protective double glass bell.
A replica of the prototype kilogram on display at Cité des Sciences et de l'Industrie, featuring the protective double glass bell. Credit: Japs 88

RG: Can you offer an insight into the new machine that is working to replace the kilogram, NIST-4?

Schlamminger: The watt balance is an instrument that compares mechanical power with electrical power. Mechanical power is the product of the weight of a mass standard times a velocity. Electrical power can be measured as a product of two frequencies, a constant and the Planck constant. So, in effect, the watt balance links the kilogram to the Planck constant. The current instrument is the fourth one that was built at NIST. We kept the things that were successful in the prior versions and we improved others that weren't so successful. An example for the former, is the wheel that we use as a balance. An example for the latter is the superconducting magnet that was used to generate the magnetic field. It was replaced with a permanent magnet in the new version.

The NIST-4 watt balance is shown. The instrument recently took its first full measurements of Planck's constant, an important step toward redefining the kilogram. Credit: Jennifer Lauren Lee/NIST PML.
The NIST-4 watt balance is shown. The instrument recently took its first full measurements of Planck's constant, an important step toward redefining the kilogram. Credit: Jennifer Lauren Lee/NIST PML.

RG: How far into the process are you? When do you hope to be finished?

Schlamminger: Redefinition of the system of units is an international process. Currently there is enough momentum and I hope the redefinition will happen in 2018.

RG: How will NIST-4 actually replace the standard kilogram? What will “Le Grand K’s” replacement look like?

Schlamminger: In the new definition of the system of units, that will hopefully come in 2018, the Planck constant will be fixed and we can use the watt balance to measure mass. Currently we are using the watt balance to measure h, by putting a known mass on it. After 2018 the process will be run in inverse.

RG: Do you think the US will ever adopt the metric system?

Schlamminger: At the highest level, the measurements are made in the metric system. The US has a national kilogram standard and not a pound standard. However, in daily use imperial units are used, which have a fixed numerical relationship to the SI. Changing the daily use to the metric system would be a huge change, think of all the tools that have to be changed (drills, wrenches, ...). In the end it is a political decision, but I am not sure if it is worth it. Especially, since it is so easy to convert between the units. So, I don't think we will see the daily use of the metric system in the USA.

Featured image courtesy of Christian Schnettelker.