THE MEASUREMENT OF RESISTANCE 



223 



As the resistance variation of copper is linear, measurements to 

 determine a tl may be made at any two temperatures which may 

 both differ from ti, the temperature of reference; for let the 

 measurements be made at temperatures tz and t 3 , then 



and 



Rt. - Rt 



R t t s - ti - 



By use of (25) and (26) the following table has been calculated. 

 The standard 100 per cent, conductivity copper is taken as hav- 

 ing a resistivity of 0.15328 ohm (meter, gram) at 20C. (see 

 page 230). 



TABLE. TEMPERATURE COEFFICIENTS OF STANDARD ANNEALED COPPER AT 

 VARIOUS TKMI-KKATURES OF REFERENCE 



Experiments have shown that distortions of the wire, such as 

 occur in winding and ordinary handling, do not alter the tem- 

 perature coefficient. 



For windings in general, annealed copper, conductivity 100 

 per cent., may be assumed. Hard-drawn copper may be assumed 

 to have a conductivity of 97.3 per cent. These values are 

 approximations to be used only when data concerning the copper 

 in question cannot be obtained. 



Measurement of Rise of Temperature. In the testing of 

 electrical machinery it is customary to find the average rise of 

 temperature of copper windings by measurements of their 



