Use of the high-voltage direct-current method for power transmission necessitates conversion of a.c. into h.v.d.c. for the line, and conversion of the d.c. back to a.c. again. The critical component of such convertors is the controllable electric valve. The basic problem of extending the voltage range of mercury-arc valves to these high levels is outlined, as well as the nature of the underlying development work. Some features of present-day valves are quoted, as well as some considerations which have to be taken into account when designing the convertor circuits, owing to the properties and limitations of actual valves.