The paper deals with mercury-arc rectifiers, mainly of the glass-bulb type, for use with shunt-wound d.c. motors to provide variable-speed drives for industrial machinery. The method is one of growing importance, and is rapidly finding new applications as its adaptability becomes more widely appreciated.1–4The characteristics considered are of practical importance to those concerned with industrial plant driven at variable speed, and to the people on whose co-operation they depend—the rectifier and the motor manufacturers. The various control methods discussed are those which have been investigated over the last 20 years; some are now obsolete but have been includedas a matter of interest.A knowledge of rectifier and motor theory is assumed, but, to assist those less familiar with rectifier practice, the Bibliography has been made reasonably comprehensive.Attention is mainly focused on those points which are peculiar to rectifier-motor operation, and which must receive special consideration in order that the rectifier, motor and machine are used together in satisfactory and economical manner.