222 ELECTRICAL MEASUREMENTS 



For a small finite temperature interval, At = t - ti, 



R t = R tl (l + a tl [t - <J). (24) 



On account of the approximations involved, equation (24) 

 applies in general only to short ranges of temperature. 



Strictly speaking, in order to compute a or one must know the 

 values of the constants a, 6, and c for the particular sample of 

 material under discussion; such data are rarely at hand. 



Special Case : Temperature Correction for Copper. Careful 

 experiments made at the Bureau of Standards 12 upon com- 

 mercial copper of high conductivity, such as is used for electrical 

 purposes (varying from 96 per cent, to 101 per cent, conduc- 

 tivity), show that between 10 and 100C. the variation of re- 

 sistance with temperature is linear, and also that the temperature 

 coefficient at 20 is directly proportional to the per cent. 

 conductivity. 

 That is, 



7? 7? 

 "20 = -4r Us = 0.00393 X (per cent, conductivity) (25) 



IvZQ I* d(J\ 



In this formula the per cent, conductivity is expressed decimally. 

 The temperature coefficient of resistance at any other temperature 

 may be calculated from that at 20 as follows, the variation of 

 resistance being linear: 



+c 



similarly 



J, -1 



a a tl 



Let the original temperature of reference, ti, be taken as 20; then 

 = 1_ 



L_ , r, _ 201 (26) 



0.00393 X (per cent, cond.) ^ 



