ELECTRICAL RESISTANCE OF NICKEL AT HIGH TEMPERATURES. 193 



corresponding to the temperatures t-r, t, t + r. The values for platinum and 

 palladium are similarly estimated. 



Temperature. 



XT A t l dJil f 



Valuesof R* Tt for 



Thick Nickel. 



Thin Nickel. 



Platinum. 



Palladium. 



50° C. 

 100° 

 150° 

 200° 



•00395 

 •00375 

 •00357 

 •00342 



•00293 

 •00306 

 •00294 

 •00294 



•00218 

 •00198 

 •00170 

 •00142 



•00302 

 •00249 

 •00225 

 •00197 



This table shows very distinctly the real nature of the difference between 

 nickel and the other two metals ; it is a difference only of degree. The quan- 

 tity R^dH/dt or d log ~R/dt, we shall, for brevity's sake, call the logarithm rate, 

 per unit rise of temperature being understood. It appears, then, that nickel 

 differs from platinum or palladium, or most other metals, in the fact that its 

 logarithm rate does not change so much with rise of temperature. In the case 

 of the thin nickel, indeed, it is practically constant, so that the march of resist- 

 ance with temperature could be very approximately represented by a simple 

 logarithmic equation. 



It may be noted that the logarithm rates for platinum and palladium are 



approximately inversely as the corresponding absolute temperatures. Hence 



we have 



_1_ dR_ h_ 



R dt~ 1 



For platinum, k = '7 roughly; 

 „ palladium, k = "95 „ 



Integrating and evaluating the constant by the condition 



R = 100 when £ = 274, 



we find, for platinum, the formula 



R = r97x*°- 7 ; 



and, for palladium, 



R = -4'83x* 095 . 



These formulas will be found on trial to be in fair agreement with the numbers 

 given in tables C and D. 



We may also by integration of 



1 dR 



R dt 



= -003 



obtain a formula for the thin nickel. Its form is 



Nap. log. (R x -0228) = -003 x t , 



