490 ELECTRICAL MEASUREMENTS 



is a disc of soft iron mounted on a fine- pitched screw, as shown 

 at X, Fig. 28 IB. The drag magnets are shielded by the cast-iron 

 frame of the instrument. 



The ability of the Sangamo meter to withstand severe mechan- 

 ical shocks and jars and its freedom from the influence of stray 

 fields, which if they do cut the armature are directed either up- 

 ward or downward on both sides of the spindle, render it appli- 

 cable to car tests in street-railway work. For this purpose a 

 special form of register, with a resetting device for registering 

 the consumption of energy during a single trip, has been devel- 

 oped. The register has also the ordinary totalizing dials. 



METER TESTING 



To maintain the accuracy of the meters in any distribution 

 system, it is necessary that they be tested periodically. On ac- 

 count of the risk of altering the constant of any form of motor- 

 meter during transportation all tests must of course be made on 

 the meters as installed. 



In many States, laws have been enacted which permit a cus- 

 tomer, in case he is dissatisfied with his bill, to request the ser- 

 vices of the appropriate public service commission in order that 

 a test may be made by a disinterested party. 



Referring to the fundamental formula for the watt-hour meter, 

 (page 461), for meters as actually constructed, the watt-hours 

 registered = Kh times (number of revolutions of disc). KH is 

 the watt-hour constant of the meter; its value is usually marked 

 on the meter disc. In some types of meters the constant is ex- 

 pressed in watt-seconds on the dials for each revolution of the disc. 

 In any case the constant K is a fixed ratio depending on the ar- 

 rangement and ratio of worm, wormwheel, gear train, and the 

 dial units of the watt-hour meter. In any meter tests which are 

 made by timing the disc as it rotates one must be certain that 

 the register used on the meter has the proper constant. 



In case of a dispute between the customer and a supply com- 

 pany the meter must be tested as found, that is before any ad- 

 justments are attempted. The records of these tests are neces- 

 sary in order that the customer and the company may arrive at 

 an understanding. 



To test a watt-hour meter, it is necessary merely to determine 



