ELECTRICITY METERS 



497 



For the greatest utility, the current range of the rotary stand- 

 ard should be large, so that it may be used for testing meters of 

 a number of different capacities. At the same time, the ampere- 

 turns due to the fixed coils must be large even when small meters 

 are tested. This insures that the standard will never be operated 

 on what is the equivalent of a light load. Therefore, the cur- 

 rent coils must be wound in sections so arranged that they can 

 conveniently be connected in various series-parallel combina- 

 tions by some reliable means. The full-load ampere-turns of all 

 the sections should be the same. 





FIG. 284. Rotating standard watt-hour meter for direct currents, 

 General Electric Co. 



Large electrical companies now use rotary standards, which are 

 accurately maintained by their laboratory departments, as sec- 

 ondary standards when checking and adjusting service meters be- 

 fore they are sent out for installation on the consumer's premises. 



While the rotary standard is very convenient and in some 



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