ELECTRICITY METERS 491 



the rate of revolution of the disc, then multiply this value by the 

 test constant of the meter, and compare the result with the 

 number of watts indicated by standard instruments which are so 

 connected in the circuit as to measure the same amount of' 

 power as the watt-hour meter under test. The energy is sup- 

 posed to be supplied at a constant rate; 



watts by watt-hour meter = P' = - - 

 For direct-current meters: 



Correct watts = P = VI; 



N = number of revolutions of the disc. 

 t = time in seconds for N revolutions. 



V = corrected average voltage measured at the potential ter- 

 minals of the meter. 



/ = corrected average current flowing through the series coils 

 of the watt-hour meter. 



If the voltage and current fluctuate badly, VI should be the 

 average watts during the test. 



If a three-wire meter, with the potential circuit connected be- 

 tween one side of the main circuit and the neutral wire, is cali- 

 brated with both current coils connected in series, then the value 

 of K h to be used in the above formulaforP' is one half that marked 

 on the disc. 



The different manufacturers of meters use various modifica- 

 tions of the fundamental formula, and one should be sure that 

 the test constant given by the maker is used in the proper manner. 

 To illustrate, for all meters made by the General Electric Co., 

 watt-hour constant = test constant. 



If "Tf ~D? ? 



Kh = Kt " g 



For Fort Wayne meters, type K, watt-hour constant = 

 test constant 

 ~36~ 



36 t 



For the Fort Wayne meters, types Ki, K 2 , K 3 , K*, watt-hour 

 constant = test constant 





