The rapidly increasing application of mercury-arc rectifiers and the certainty of their exclusive use in the near future in the bulk conversion of alternating to direct current for railway electrification projects, require that more careful consideration be given to the peculiar nature of the load which a rectifier imposes on the supply system. Few electricity supply authorities appreciate that, in the case of rectifier substations, power-factor meters give no real indication of the additional extent to which the generating plant is loaded.The paper investigates the manner in which the mercury-arc rectifier draws power from the supply, with the object of ascertaining the degree to which the generation and distribution system is utilized.The general case of power production from currents and voltages of non-sinusoidal wave-form is considered, and, by introducing a new factor, viz. harmonic power, fresh significance is given to the conception of wattless power.The presence of harmonics in the current delivered by and supplied to the rectifier is briefly explained, and their modification due to smoothing equipment and to parallel connection of rectifiers is indicated.In dealing with the power-factor problem as applied to the mercury-arc rectifier, its treatment by previous investigators is reviewed. The relation between rectifier and system power factors is then discussed and an analytical method of determining the additional loading due to harmonics is developed. Convenient formulae for evaluating the resultant power factors and for expressing the harmonic loading in terms of the reactive power are finally deduced.The paper concludes with a suggested application of the foregoing method of analysis to the amendment of electricity supply tariffs.A bibliography of the available literature covering the problems dealt with in the paper is given at the end of the paper.