The paper discusses briefly the principal features of the mercury-vapour rectifier, and describes a number of novel and successful applications of the use of automatic and semi-automatic mercury-vapour rectifier substations, designed for developing an efficient and economical direct-current supply to areas remote from existing sources of supply.A method is given of feeding into an existing low-tension direct-current network by means of automatically controlled rectifiers for the purpose of improving the voltage and assisting already overloaded feeders. A specific case is considered, and the cost of three methods of dealing with this particular case is analysed.The three methods considered are:-(1) By means of new low-tension feeders from existingsubstations.(2) By means of one new manually-operated substationand new low-tension feeders.(3) By means of a number of automatic mercury-vapourrectifier substations.Arrangements for the automatic and semi-automatic control are given, together with a description of the means adopted for automatically regulating the d.c. voltage.A method of giving a d.c. supply at 550 volts to a 6-mile length of double-sleeper tramway track by means of semiautomatic rectifier substations is described, and complete details are given with the cost of one of these substations.Sites and buildings for each of these types of substations are discussed, and general lay-outs are given, with special reference to simplicity, safety and cheapness.