506 ELECTRICAL MEASUREMENTS 



water rates of three-phase turbo-generators, water rheostat loads 

 being employed. The three-phase power factor is then unity. 

 In calibrating the meter, together with the transformers, a three- 

 phase non-inductive load should be used and the connections so 

 made that the element which operated with the lagging power 

 factor during the test is traversed by a lagging current during 

 calibration. 



Testing of Large Direct-current Watt-hour Meters on Fluc- 

 tuating Loads. 15 On account of the great revenue per meter 

 which may be involved, it is very important for both the supply 

 company and the consumer that the meters by which large 

 amounts of power are sold be kept in an accurate condition. 

 The necessary tests must be made with the meters in place, and 

 if they are used on a rapidly fluctuating load, such as a street- 

 railway system, difficulties are experienced in making the tests 

 and the necessary adjustments. 



Owing to the large number of readings of the current which 

 it is necessary to take in order to obtain a good average, the ordi- 

 nary method of using a stop-watch and of measuring the line 

 voltage and current is a time-consuming operation, and in some 

 cases the fluctuations are so rapid that the use of the ammeter 

 is quite out of the question. An alternative procedure is to take 

 the meter out of service and to send through its coils the current 

 from a storage battery (see page 500). This current may be 

 controlled by resistors, so that tests at light load and up to about 

 500-amp. may be made without the apparatus being too un- 

 wieldy to be managed by two persons. For this purpose two 

 Edison cells are convenient, being readily portable. It is, how- 

 ever, desirable to avoid taking the meter out of service, for the 

 test may occupy an hour or more, and the loss of revenue is worth 

 obviating; it may be as much as $5 to $10 for each hour the meter 

 is out of service. Also it is desirable to make the test with the 

 customer's regular load. 



The very convenient portable standard watt-hour meters de- 

 veloped for alternating-current work naturally suggested similar 

 devices for use on direct-current circuits. Their development, 

 however, has been attended with much difficulty. Nevertheless 

 the problem has been solved quite successfully, and the best of 

 these instruments, when carefully used, are of great service where 



