406 



TABLE 417.— TABULAR COMPARISON OF WIRE GAGES (concluded) 



TABLE 418.— TEMPERATURE COEFFICIENTS OF COPPER FOR DIFFERENT 



INITIAL TEMPERATURES (CENTIGRADE) AND DIFFERENT 



CONDUCTIVITIES 



Note.— The fundamental relation between resistance and temperature is the following : 

 R, = R, 1 (l + a, 1 [t — t l ]), 



where a (l is the "temperature coefficient," and h is the "initial temperature" or "tempera- 

 ture of reference." 



The values of a in the above table exhibit the fact that the temperature coefficient of 

 copper is proportional to the conductivity. The table was calculated by means of the 

 following formula, which holds >for any percent conductivity, n, within commercial ranges, 

 and for centigrade temperatures, (n is considered to be expressed decimally : e.g., if 

 percent conductivity = 99 percent, n = 0.99.) 



1 



o« 1 — z 



+ (tt-20) 



n(0.00393) 



SMITHSONIAN PHYSICAL TABLES 



