130 



ELECTRICAL MEASUREMENTS 



in which case the coefficient applying to the particular coil in 

 hand must be employed. 



30 32 34 36 



FIG. 58. Showing effect of temperature on a manganin resistance coil. 



10 12 U 16 18 20 22 24 26 

 Degrees Centigrade 



The possible effects of a rise of temperature on a resistance 

 coil are: 



1. If the rise of temperature is small there will be a temporary 

 change in the resistance; that is, one which disappears when the 

 normal temperature is regained. 



2. If the rise of temperature is great there will be a per- 

 manent alteration of the resistance. 



3. At a still higher temperature the insulation will be impaired. 

 It is evident that the ability of a resistance coil to dissipate the 



heat due to the passage of the current is most important, and as 

 the precisions which may be obtained with various methods of 

 measurement are proportional to the currents employed, it is 

 advisable, when selecting a resistance box for general use, to 

 choose one having coils of a high watt capacity. 



In any case the safe working current is that which will heat the 

 coil to a temperature just below that at which a permanent 

 alteration, or "set," in the resistance will take place. A coil 

 adjusted to 0.01 per cent., which is expected to maintain this 

 degree of reliability, must be more carefully treated than one 

 certified to 0.05 per cent, which is a degree of adjustment fre- 



