ELECTRICITY METERS 493 



Common Sources of Inaccuracy. If the meter is very slow, 

 or cannot be brought up to speed, the trouble may be due to: 



1. Commutator and brushes pitted, oily and dirty. 



2. Commutator segments short circuited. 



3. Lint or magnetic particles between drag magnet and disc. 



4. Disc may not run true, or may be out of position. 



5. Pivot worn. 



6. Jewel rough or cracked. 



7. Dirt in jewel. 



8. Undue friction in the worm and the registering train. 



9. Upper guide bearing pressed down on shoulder of spindle. 



The meter may register too much, due to weakening of the 

 magnets, through ageing or by a short circuit on the customer's 

 (premises. 



Methods of Testing. There are several methods of making 

 tests to determine whether the meter is registering correctly or 

 not. They differ in the arrangement employed for ascertaining 

 the true amount of power or energy delivered to the load via the 

 meter. The arrangements commonly used for this purpose are: 



1. Indicating instruments. 



2. A calibrated load box together with a voltmeter. 



3. A rotary standard watt-hour meter. 



When indicating instruments or a calibrated resistance are 

 employed, the time in seconds required for a whole number of 

 revolutions of the moving element of the watt-hour meter is 

 determined by means of a stop watch. In direct-current work, 

 calibrated ammeters and voltmeters of the moving-coil type are 

 used to determine the true watts. In alternating-current work 

 a calibrated indicating wattmeter is used. If small meters are 

 tested, one must be sure that the results are not complicated by 

 the loss occurring in the voltmeter or in the potential coil of the 

 indicating wattmeter. The watts given by the meter are calcu- 

 lated by the appropriate test formula and compared with the 

 results li-iven by the indicating instruments. "The percentage 



meter watts inn rp, ra t A n f thp 



of accuracy" is given by , - X 100. I he rate oi tne 



true watts 



meter watts 

 meter is given by 



