A precision electrodynamometer standard and a.c./d.c. transfer instrument

A description is given of the design, construction and development of a precision wattmeter-voltmeter-ammeter-milliammeter and a.c./d.c. transfer instrument used at the South African National Physical Laboratory for testing a.c. instruments. When used as a wattmeter, ammeter and voltmeter the instrument has current ranges of 0.5¿5 amp, and voltage ranges of 25¿500 volts. The milliammeter range is 50 mA. Results obtained with this instrument are accurate within 0.01% of full-scale value on either direct or alternating supplies of 50 c/s, and at power factors between unity and 0.5. Corrections have been established to permit of precise measurement at higher frequencies or at lower power factors.