Sosman — Platinum- Rhodium Thermoelement. 3 



In our revision of the nitrogen scale from zinc to palladium,* 

 one measurement was made at the melting point of cadmium, 

 to give an indication of the course of the thermoelement curve. 

 Being only a single measurement, this has not as much weight 

 as the higher temperatures, which were measured under varied 

 conditions. The value obtained was 320"0°. The difference 

 between benzophenon and cadmium, determined with three 

 thermoelements, is 14"9°, which agrees within 0'1° with a 

 similar comparison made at the Bureau of Standards, using a 

 resistance thermometer and different samples of the materials. 



On tlie basis of the benzophenon value adopted above, this 

 difference makes the cadmium point 320-3°. We have arbitra- 

 rily connected the two portions of the temperature scale at this 

 point by taking the mean, 320"2°, for cadmium. Since in this 

 region temperatures cannot be conveniently obtained closer 

 than 0*2° with the platinum-rhodium element, the values are 

 abundantly accurate for our present purpose. 



From zinc to palladium, we have used, throughout, the scale 

 of temperatures published in our recent paper.f 



It should not be overlooked that the value which we have 

 used for zinc indicates a lower value for the boiling point of 

 sulphur than the figure 444"5 now in general use. The four 

 independent gas thermometer determinations that have been 

 made of the sulphur point, although agreeing unusually well, 

 are not free from the possibility of errors of several tenths of 

 a degree, and this fact, taken together with the variability in 

 tlie point itself with different experimental conditions, makes 

 it probable that the absolute value given for the sulphur point is 

 at least no more reliable than the value given below for the 

 zinc point. 



3. Melting Point of Platinum. 



For the purpose of interpolating between the melting points 

 of palladium and platinum, it was necessary to determine, on 

 a number of thermoelements, the thermal e.m.f. at the latter 

 temperature. This was done by heating the element up to 

 the melting point of the platinum wire, within a glazed Mar- 

 quardt porcelain tube, in the region of maximum temperature 

 of a resistance furnace. The furnace was of the carbon tube 

 type.:}: Carbon monoxide around the outside of the porcelain 

 tube protected the furnace from oxidation, and a current of 

 dry air in the inside prevented contamination of the thermo- 

 element. Both wires of the element were enclosed in Mar- 

 quardt capillaries, leaving only about 2""" of the platinum 

 exposed next to the junction. It was always this portion that 



* Loc. eit. 



f Loc. eit., p. 161. 



is. A. Tncker, Trans. Amer. Electrochem. Soc. xi, 303, 1907. 



