194 Prof. H. L. "Callendar on Platinum Thermometry. 



metals, according to Benoit, is approximately represented by 

 an empirical formula of the type 



R/R°=l + at + bt 2 , (B) 



where B is the resistance at any temperature t, and R° the 

 resistance at 0° C. The values of the constants a and b which 

 he gives for iron and steel represent correctly (in opposition 

 to the formula of Siemens) the very rapid increase in the rate 

 of change of resistance with temperature, as shown by the 

 relatively large positive value of the coefficient b. He gives 

 also in the case of platinum a small negative value for b (a 

 result since abundantly confirmed), although the specimen 

 which he used was evidently far from pure*. This formula, 

 which is the most natural to adopt for representing the 

 deviations from lineality in a case of this kind, had been 

 previously employed to a limited extent by others for the 

 variation of resistance with temperature; but it had not pre- 

 viously been proved to be suitable to represent this particular 

 phenomenon over so extended a range. 



The work of the Committee of the British Association in 

 1874 was mainly confined to investigating the changes of 

 zero of a Siemens pyrometer when heated in an ordinary fire 

 to moderately elevated temperatures. Finding that the pyro- 

 meter did not satisfy the fundamental criterion of givinc 

 always the same indication at the same temperature, it did. 

 not seem worth while to pursue the method further, and the 

 question remained in abeyance for several years. In the 

 meantime great advances were made in the theory and 

 practice of electrical measurement, so that when I com- 

 menced to investigate the subject at the Cavendish Labo- 

 ratory, the home of the electrical standards, in 1885, I was 

 able to carry out the electrical measurements in a more 

 satisfactory manner, and to avoid many of the sources of 

 error existing in previous work. The results of my investi- 

 gations were communicated to the Boyal Society in June 

 1886, and were published, with additions, in the ; Philoso- 

 phical Transactions ' of the following year. Owing to a 

 personal accident, no complete abstract of this paper as a 

 whole was ever published ; and as the paper in its original 

 form is somewhat long and inaccessible, many of the points 

 it contained have since been overlooked. The greater part of 

 the paper was occupied with the discussion of methods and 

 observations with air-thermometers ; but it may not be amiss 

 at the present time to give a summary of the main conclusions 



* It may be remarked that the sign of this coefficient for platinum and 

 palladium is wrongly quoted in Wiedemann, JSlectricitat, vol. i. p. 525. 



