A preliminary survey is made of the difficulties involved in metering (a) active energy, wattless kVA, total kVA, and power factor, in the input circuits of mercury-arc rectifiers, owing to the presence of harmonics in current and voltage;and (b) energy output from mercury-arc rectifiers when considerable ripple exists in the current and voltage.The performance of induction-type and motor-type integrating meters is then examined experimentally on rectifier circuits, and the observed errors are quoted. Good agreement is found between the observed errors and those expected from theory and the known variation of the meter characteristics with frequency, both in the case of energy and in the case of kVA measurement. Similar tests are described on motor- and mercury-type integrating meters on the output side of rectifiers.It is concluded that a.c. power measurements can be made with satisfactory accuracy by induction- and motor-type meters. No entirely satisfactory solution is found, however, for power-factor and kVA measurement. Finally, the necessary precautions to be taken with d.c. integrating meters on the output side are dealt with.