ELECTRICITY METERS 



463 



blocks so that they cannot alter their positions or draw together 

 and crush the armature if a short-circuit occurs. 



As friction losses in the instrument are unavoidable, they must 

 be reduced to a minimum and must remain practically constant 

 over long periods of time. Therefore the moving parts should 

 be light, the jewels and pivots of the best, and kept in good order. 

 The jewels should be carried by spring supports as this construc- 

 tion increases the life of the jewel by the elimination of "hammer- 

 ing" and consequently assists in maintaining accuracy at light 

 loads. 



<-Shaft 



FIG. 267. Jewel supports for watt-hour meters. 



Cupped diamond jewels are now used in direct-current meters 

 of large capacity; this contributes materially to the maintenance 

 of accuracy at light loads. 



In order to reduce the wear on the moving parts of a meter, 

 the full-load speed is limited to about 50 revolutions per minute. 



The commutator should be of small diameter and smooth 

 and must be kept free from oil and dust of all kinds; the brushes 

 must be smooth and the brush pressure properly adjusted. 



The counter should have the minimum possible friction and 

 the worm and wheel connection to the armature spindle should 

 be properly adjusted. Steady pins should be used to insure 

 permanence of the adjustment. 



The torque of the meter should be high so that the unavoidable 

 irregularities in friction may not cause inaccuracies, for in time 

 the pivot and jewel as well as the commutator become rough, es- 

 pecially if the meter be subject to vibration and sudden jars. 



