Wheatstone bridge — Rotating standard method of testing large capacity watt-hour meters

There are two general classes of large capacity watt-hour meters, alternating-current and direct-current. The meters used for alternating current are almost invariably of the induction type, generally five-ampere meters used in conjunction with current transformers, or both current and potential transformers. A facile and accurate method of testing these, meters is by means of an induction type of rotating standard which by means of standard instrument transformers may be made available for testing any a-c. meter of any capacity, with a precision well within commercial limits.