i_A] SERIES GENERATOR. " 



t C. At a higher temperature the resistance will be greater and 

 experiment shows that the increase in resistance will be in direct 

 proportion to the temperature rise. 



At a temperature (t + 0) C. the resistance is accordingly 



The temperature coefficient a (per degree C.) depends upon the 

 initial temperature t (degrees C), or the temperature for which the 

 resistance is taken as 100 per cent, and has for copper the following 

 values :* 



/ o 6 12 18 25 32 40 48 

 a .0042 .0041 .0040 .0039 .0038 .0037 .0036 .0035 



From the formula given above, if the resistance is known for one 

 temperature, the resistance can be computed for any other tempera- 

 ture or for any temperature rise. 



1 6. From this formula we can also compute the temperature rise 

 6, above the initial temperature t, corresponding to a known increase 

 in resistance. By transformation the formula becomes 



The temperature rise above an initial temperature / is accordingly 

 equal to the per cent, increase in resistance divided by a. 



17. Fall of Potential Method for Measuring Resistance. This 

 method is based upon the fact that the fall of potential through a 

 resistance R carrying a current / is E = RI (Ohm's Law). The 

 resistance R which is to be determined may be the resistance of any 

 conductor whatever (transformer coil, field winding, armature, etc.) 

 which will carry a measurable current without undue heating and 

 is not itself a source of electromotive force. An armature, there- 

 fore, must be stationary while its resistance is being measured by 

 this method. 



Connect the unknown resistance to a source of direct current 

 through a regulating resistance, Fig. 6 (see also 14), so that the 

 current will not unduly heat the resistance or exceed the range of 

 instruments. Take readings of the two instruments simultaneously, 



*A. I. E. E. Standardization Rules; also, A. E. Kennelly, Electrical 

 World, June 30, 1906. 



