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.