An account is given of the equipment used and the methods which have been developed at the National Physical Laboratory for calibrating alternating-current indicating instruments to an accuracy of 0.1%, or better, of full-scale deflection. The power required for carrying out these tests may range from a few watts for a low-range voltmeter or ammeter up to 60 kW for the equipment used in a turbo-alternator test at high voltage. When the power required is large and at a frequency below 600 c/s, rotating machinery is used, giving an output stable to 0.02% and of waveform with harmonic content not exceeding 2%. For smaller powers, oscillator-amplifier sets are used at all frequencies from 50 c/s up to lOOkc/s, giving an output stable to 0.01% and of waveform with harmonic content not exceeding 0.3%.Instruments are tested by comparison with a standard voltmeter or wattmeter which must itself be calibrated to a higher order of accuracy, say 0.01 or 0.02%. The auxiliary resistors used with these instruments for range changing must also maintain constant and known values under all loading conditions to about 0.01%. Finally, the instrument, a voltmeter, used as a d.c./a.c. transfer instrument must be calibrated on direct current against a Weston cell in association with a suitable voltage divider. The e.m.f. of the Weston cell is maintained constant to approximately 0.001% by maintaining the cell at a temperature constant to 1/4° C, while the error of the voltage divider is of the same order, so that no significant error is introduced into the calibration on account of the d.c. standard.