ELECTRICITY METERS 



509 



balance is not affected by stray fields. Two resistors, one in the 

 circuit of the test meter, and the other in the parallel circuit, are 

 employed. The potential drops in the two are made equal by 

 the adjustable resistor r and this equality is indicated by a differ- 

 ential millivoltmeter of the D'Arsonval pattern. Any shunts 

 which are suited to the purpose may be temporarily bolted 

 together and used for Si and $ 2 . They should be free from ther- 

 mal errors as these are troublesome in some cases. 



The arrangement may be simplified and the differential milli- 

 voltmeter replaced by a pivoted D'Arsonval galvanometer if a 

 special double shunt be constructed for the purpose. The con- 

 nections are shown in Fig. 297. Inspection of the figure will 



FIG. 297. Connections for testing large direct-current watt-hour meters 

 by the bridge method. 



show that the arrangement has become a Wheatstone bridge, the 

 low-resistance sides being composed of fixed resistors. One of 

 the high-resistance sides is a resistor of fixed value; the other is 

 made up of the rotary standard and the necessary adjustable 

 resistor for maintaining the bridge in balance when contact and 

 coil resistances change. The sections of the shunt (see Fig. 298) 

 Si and S 2 have a common terminal at a. If the galvanometer 



T ^f 



stands :ifr, /cro, then ~ = cA and the corrected reading of the 

 lz 1 



test meter is its indication multiplied by -^-j -. By the use of 



