446 Mr. J. D. Hamilton Dickson on 



In 1887 Professor Callendar*, as the result of an elaborate 

 and careful experimental investigation of the possibility of 

 indicating temperature by the variation of the resistance of 

 platinum, devised the idea of " platinum-temperatures," given 

 by the formula 



R— Rq 



R, — R 



100, (3) 



where zr is the " platinum-temperature " corresponding to the 

 resistance, R, of a given wire whose resistances are Rj and R 

 at 100° C. and 0° C. To this formula he added the correction- 

 formula 



*-^{(mJ-m} <■* 



by which ■bt is to be reduced to "normal air-temperature" t, 

 8 being a constant to be determined once for all for each wire. 



Further researches since then by Mr. E. H. Griffiths f, by 

 Prof. Callendar and Mr. Griffiths^, by Professors Dewar and 

 Fleming §, by Messrs. Heycock and Neville ||, and by Messrs. 

 Holborn and Wien %, go to show that, for temperatures 

 ranging from — 200° C. to 1300° C, however roughly a 

 given platinum wire may be treated as regards rapid or ex- 

 cessive variations of temperature or long immersions in regions 

 of either very low or very high temperatures, provided it be 

 carefully annealed, it undoubtedly always has the same resist- 

 ance at the same temperature. 



Mr. E. H. Griffiths further found (Gr.) that two different 

 platinum wires, although giving different values (each on its 

 own scale) of the platinum-temperature corresponding to a 

 given normal air-temperature, nevertheless agreed, when pro- 

 perly corrected, in giving the same value for the normal 

 air-temperature. 



Finally, Professors Dewar and Fleming** showed experi- 

 mentally that for pure metals the resistance vanished at or 

 near the absolute zero, but that the temperature-coefficient, 

 which Clausius hoped might be '00366 for all, varied in each 

 case. 



* Trans. Roy. Soc. 1887 A ; in the present paper referred to as (Call.), 

 t Trans. Roy. Soc. 1891 A ; in the present paper referred to as (Gr.). 

 % Trans. Roy. Soc. 1891 A; in the present paper referred to as (C. and 

 Gr.). 



§ Phil. Mag. 1895, vol. xl. p. 97, § 4. 

 || Trans. Uhem. Soc. 1895. 

 II Wied. Ann. vol. lix. (1896). 

 ** Phil. Mag. vol. xxxiv. (1892). 



