238 



ELECTRICAL MEASUREMENTS 



perature, the major portion of the resistance must be of a 

 material having a negligible temperature coefficient. In addi- 

 tion, the effect of temperature on the controlling springs, and in 

 direct-current instruments the effect on the magnets, must be 

 small. The springs grow weaker by about 0.04 per cent, per 

 degree C. as the temperature is raised. Usually the magnets 

 grow weaker with an elevation of temperature, an average value 

 for the temperature coefficient being 0.025 per cent, per degree 

 C. The net effect of temperature on a 150-volt direct-current 

 portable voltmeter of good design is about + 0.012 per cent, 

 per degree. 



FIG. 130. Multipliers for extending the range of voltmeters and wattmeters. 



The effect of temperature variation becomes more important 

 when low-range instruments are used for in them the movable coil, 

 which is always wound with copper wire, forms a relatively larger 

 proportion of the total resistance. 



Multipliers. Frequently it is necessary to measure potential 

 differences higher than those for which the voltmeter was origi- 

 nally intended. In this case a properly constructed resistance 

 is joined in series with the instrument so that the voltage 

 necessary to force a given current through the voltmeter circuit 



