498 



ELECTRICAL MEASUREMENTS 



cases necessary, one must not forget that great care must be 

 taken if accurate results are to be obtained. 



The direct-current instrument is heavier and less convenient 

 than that for alternating current, and is not so commonly used. 

 When the instrument is calibrated, and when it is used, it is 



FIG. 285. Rotating standard watt-hour meter for alternating currents, 

 Westinghouse Co. 



necessary to keep the potential-coil circuit of a direct-current 

 rotary standard energized for a considerable time (about 30 

 min.) before any readings are taken long enough for the entire 

 armature circuit, the disc, and the drag magnets to attain their 

 permanent states of temperature, since the resistance of the 

 armature circuit, the resistance of the disc to eddy currents and 



