ELECTRICITY METERS 



499 



the strength of the drag magnets are all dependent upon tem- 

 perature. The heat liberated in the current coils also influences 

 the accuracy of the meter for it, too, affects the temperature of 

 the potential coil, the disc and the drag magnets. This self- 

 heating error may be of importance in careful tests if the meter is 

 so used that it must carry a large current for a long time. The 

 alternating-current standard is subject to the errors found in 

 meters of the induction type (see pages 478, 505). Therefore, 

 in case of a serious dispute between the consumer and the supply 

 company, it is preferable to use indicating instruments if possible. 



Source 



FIG. 286. Timing device for calibrating watt-hour meters. 



The use of rotary standards takes the determination of the 

 time element from the tester, who must of necessity use a stop 

 watch, and hands it over to the laboratory department, where 

 much more accurate timing devices may be maintained. 



A timing device designed for use in calibrating rotary stand- 

 ards is shown diagrammatically in Fig. 286. 



The master clock which operates the device has a pendulum 

 which beats seconds (% period). At each beat of the pendulum 

 the relay contact is made and the ratchet wheel is advanced 

 one tooth, carrying with it the contact sector e. The duration 

 of the contact of the spring b corresponds to 36 teeth on the rat- 

 chet wheel, in other words, to 36 sec. The potential circuit, 



