The development of a sensitive precision wattmeter for the measurement of very small powers

The need and requirements for a wattmeter suitable for the measurement of iron losses, and capable of measuring powers of the order of 0.1 watt at low power factors with an accuracy of at least 1 per cent, are discussed. The principle of the suggested instrument is given, and the various errors likely to occur and methods of eliminating them are reviewed. The factors governing the design, and the method of design, follow. The results of seven designs are tabulated, the best being chosen. Finally the details and results are given of the tests carried out on a completed instrument.