16 



Prof. T. S. Humpidge. 



The agreement between the calculated and observed results is good, 

 except in the last two instances. The selenium probably contained 

 some sulphur, which would lower its boiling point, and only one 

 experiment was made with zinc, as this temperature could only be 

 reached in the furnace used under an exceptionally favourable gas 

 pressure. 



The change in the resistance of commercial platinum has been 

 investigated by the late Sir W. Siemens, M. Benoit, and others. 

 Siemens expressed his results in the form — 



n = r„(AT* + BT-C), 



and this can be thrown into a general expression similar to that 

 which I have used, viz. : — 



n = r (l + ed - fit 2 ). 



Benoit's results* can also be expressed by a similar formula. 



But although there is a general agreement between these different 

 results, the values of the coefficients vary considerably, and it is 

 therefore always necessary in using this pyrometer to calibrate it for 

 every wire. I have even found that the change of resistance of two 

 pieces of wire from the same bobbin varied considerably. 



M. Benoit has kindly furnished me with the following synopsis of 

 various results, which, although they differ much from one another, 

 can all be expressed by the general formula given above : — 







Siemens. 

























t. 



i: 



II. 



III. 



Benoit. 



Erhardt.f 



Humpidge, 



0°.. 



1-00 



1-00 



1-00 



1-00 



1-00 



1-00 



100 .. 



1-29 



1-25 



1-31 



1-23 



1-23 



1-27 



200. . . 



1-47 



1-50 



1-62 



1-46 



1-46 



1-53 



400 .. 



1-84 



1-98 



223 



1-91 



1-90 



2-05 



600 .. 



216 



2-48 



2-81 



2-34 



2-31 



2-55 



800 .. 



2-43 



2-97 



3-37 



275 



2-69 



3-04 



* Compt. rend., lxxvi, 342. The formula is incorrectly given in Wiedemann's 

 " Electricitat," the sign of second coefficient being positive instead of negative. 

 One of the temperatures used in this research was that of boiling cadmium, which 

 was taken from the earlier and incorrect determinations (with an iodine thermo- 

 meter) of Deville and Troost as 860° instead of circa 770°. M. Benoit has, 

 however, informed me, in a private communication, that on introducing this cor- 

 rection the general form of the equation remains the same. 



f " Wied. Ann." xxiv, 215. 



