Ezer Griffiths’s research while affiliated with National Physical Laboratory and other places

The relationship between relative humidity and the mobility of negative ions in air at atmospheric pressure has been examined with a view to possible applications in hygrometric measurements. The method employed was a modification of Zeleny's original method, the end of a wind channel being closed by a disc of gauze fitted with a guard ring through which a steady stream of air of definite humidity was pumped. The motion of the negative ions due to the action of the air stream was balanced by a counter potential gradient, and the mobility was deduced from the critical potential required to produce a balance. The rate of air flow was measured by means of a Ewing ball and tube flowmeter, a hollow glass sphere being used to make it suitable for low air rates. Efforts were made to construct a direct indicating instrument by amplifying the current sufficiently to obtain deflections on a robust moving-coil indicator, but the variations in the amplification factor were found to be too serious.

The relation between relative humidity and wet-and-dry-bulb readings, at dry-bulb temperatures between 40° and 100°C., has been studied experimentally. The relative humidity was measured both by absorbing and weighing the moisture in a known volume of air, and by the dew-point method. The results are finally expressed by means of a skeleton table for the wet-and-dry-bulb hygrometer in the temperature range referred to.

The specific heat of liquid methyl chloride has been determined over the range -30° to +30° c. The liquid was in a sealed container provided with a heating coil in a re-entrant tube. Stirring was effected by means of an external solenoid operating on an iron armature. The calorimeter was surrounded by an adiabatic enclosure. The quantity determined experimentally included the latent heat of the liquid vaporized and the heat supplied to warm the vapour in the calorimeter. This was reduced to give the specific heat of the liquid following saturation. Comparison of data with those of previous investigators indicates good agreement with those of Eucken and Hauck at -30° c. and the extension of the line representing their results over the range from -80° to -30° c. The results given by Shorthose are slightly higher.

The viscosities of sulphur dioxide, ethyl chloride, methyl chloride, cis-dichlorethylene, trans-dichlorethylene, trichlorethylene and dichlor-difluormethane have been measured at temperatures between -15° and +30°C. The method used is that of timing the rate of fall of a closely fitting plug in a vertical tube filled with the liquid. In order to avoid evaporation, with the resultant formation of bubbles, an auxiliary reservoir of the fluid was connected with the experimental tube; the refrigerant in the reservoir was maintained at a temperature slightly above that of the tube, so that the pressure in the apparatus was above the vapour pressure of the liquid in which the plug moved. In all cases the viscosity η was found to follow a law η = Aea/T where T is the absolute temperature and A and a are constants.

The Paper describes a robust form of the Ewing Ball and Tube Flowmeter. suitable for the metering of gases or liquids under pressure, as, for example, the ammonia in a refrigerating plant. The necessary pressure-tight joints for connecting the conical tube to the circuit are described, and also a device for cutting off the flow should the tube fail. An investigation is included of the behaviour of the instrument under practical conditions of pulsating flow.

The latent heats of a number of the commoner metals have been measured by determining the total heat of liquid and solid from a series of initial high temperatures. The calorimetry was by the method of mixtures, introducing several refinements, of which the chief were the use of fairly large charges of metal (of the order of 2 kilograms), and a device by which the hot charge was not allowed in contact with the water of the calorimeter until the latter was completely closed; this eliminates error due to production and escape of steam, with a consequent loss of heat. The device referred to consisted in the provision of a sheet metal vessel suspended by threads from the main lid of the calorimeter. The aperture through which the charge was introduced was closed by a rotating lid, in the main lid, and the crucible being introduced, was submerged after the closing of this smaller lid by means of a wire passing through an eyelet in the base of the calorimeter, and out at the top. The results for the latent heat are given below: - Metal. Melting point, °C. Latent Heat. (Calories per gm.) Aluminium 657 92.4 Antimony 630 24.3 Bismuth 269 13.0 Lead 327 6.26 Magnesium 644 46.5 Tin 232 14.6 Zinc 420 26.6 The Paper also contains values for the specific heats up to the melting point, obtained by differentiation of the temperature-total heat curves.

Two new forms of apparatus for determining the heat of evaporation of liquids of high boiling point are described, and their relative merits briefly discussed. In the one the determination is made by a measurement of the amount of liquid vapourised for a given energy supply (radiation loss being eliminated by means of a constant temperature jacket) and in the other, the energy given up on condensing is estimated by means of a continuous flow calorimeter. The latter is thus a modification of the well-known apparatus of Berthelot, but particular attention has been paid to the avoidance of defects inherent in earlier types of apparatus. The apparatus have been tested by determining with them the heats of evaporation of such liquids as alcohol, water, and aniline, whose heats of evaporation are well known. A brief survey of the earlier work is included, and a number of references given.

The humidity corresponding to various wet-and-dry-bulb temperatures between -2° C. and -19° C. has been measured. The wet-bulb temperatures were obtained by means of mercury thermometers, thermocouples and resistance thermometers, all three methods agreeing well. The actual humidity was generally obtained from a specially designed dew-point apparatus, thermojunctions being used to measure the temperature of the metal surface on which the deposit of dew was formed. In addition, tests by the gravimetric method were carried out. The results are utilized to prepare a table giving the relative humidity at various wet-and-dry-bulb temperatures. These agree, for the most part, with previous tables, but they differ in the region of low dry-bulb temperatures (below about -9° C.) and small wet-bulb depressions (less than 1° C.). The results are also examined from the point of view of the usual theory which asserts that (e'−e) = B0P (θ − θ'), where e' is the saturation vapour pressure at the wet-bulb temperature θ', e the saturation vapour pressure at the dew point (i.e. the actual partial pressure of water in the air), θ the dry-bulb temperature, P the barometric pressure and B0 the psychrometric constant. Although above 0° C. this formula is found to hold well, yet below 0° C. it is found that B0 varies in a complicated manner with the temperatures. It tends to infinity when the wet-bulb depression tends to zero, and there is a further variation superimposed on this one; at a constant value of the depression, B0 passes through a minimum and then through a maximum as the dry-bulb temperature decreases.