Platinum Metals Rev., 1957, 1, (4), 131
Special Platinum Alloy Thermo-Junctions
RAPID MEASUREMENT OF THERMAL CONDUCTIVITY
A platinum alloy thermocouple junction which closely resembles a single wire, uniform in size, in electrical resistivity and in thermal properties from end to end has recently been developed and produced by the Research Laboratories of Johnson Matthey & Co. Limited in collaboration with Mr. R. A. W. Hill, of the Research Department of the Nobel Division of Imperial Chemical Industries Limited, for use in a new method of measuring the thermal conductivities of poor conductors.
As described by Mr. Hill in Proc. Roy. Soc. A, 1957, 239, 476-486, the principle of the method is to observe the rate of heating of a fine wire when a constant radio-frequency current is passed through it, the wire being maintained first surrounded by air and then surrounded by the liquid or other medium of which the thermal conductivity is to be measured. By comparing the heating curves of the wire in the two environments, the thermal conductivity of the medium can be determined. The measurement can be made in less than one-tenth of a second.
In order to measure its temperature, the wire is made in the form of a thermo-junction of two 40 swg wires chosen to have the same resistivity, and the thermo-e.m.f. is observed by an oscillograph.
The alloys used for the two elements of the thermo-junction were finally chosen, after experimenting with a number of combinations, as (a) platinum with 8 per cent gold and (b) platinum with 2 per cent ruthenium. The electrical resistivities of these alloys are closely matched and the couple develops about 15 microvolts per degree C. The couple wires were welded together and the junctions drawn down in a die to the diameter of the wire so as to be quite invisible to the eye. By this means, it has been found possible to apply this method to small, rapidly changing systems at high temperatures. The accuracy has been checked by measurements on medicinal paraffin, glycerol, and water.