ELECTRICITY METERS 



507 



load conditions are extremely variable. Such test meters are 

 now made in capacities up to 150 amperes. 



In railway work, it is frequently necessary to test meters of 

 several thousand amperes capacity. The direct application of 

 shunts to a portable standard watt-hour meter, of the commuta- 

 ting type, is not permissible on account of the change of the multi- 

 plying power of the shunt through heating, and the uncertainty 

 due to bad contacts. 



As it is desirable to retain this type of meter as a standard, 

 methods have been devised whereby shunts are applied to the 

 portable standard watt-hour meter in such a way that errors due 

 to heating and to contact resistances are eliminated. 





FIG. 295. Showing connections for testing large direct-current watt-hour 

 meters by the differential multiplier method. 



One method is shown in Fig. 295, where for the sake of sim- 

 plicity the potential connections to the meters are omitted. The 

 arrangement may be called a differential multiplier, for by its use 

 the range of the portable standard watt-hour meter is extended. 



The station busbar is arranged so that it has a narrow gap at G. 

 This gap is ordinarily closed by plates firmly bolted in position. 

 The gap should be narrow and the leads so arranged that the field 

 at the meter is not altered when the gap is opened. The test 

 circuit is clamped to the bus bars and the gap opened without in- 

 terrupting the service. The entire current then flows to a, where 

 it divides, a comparatively small portion flowing through the fine 

 wire coils of the multiplier MM' , the rotary standard and the 



