A regulator of simple construction is described, which is capable of maintaining, for long periods of time, an accurately constant "root mean square" voltage in the circuit of an electric resistance furnace. It is shown that, in ordinary circumstances, a laboratory furnace of medium size, controlled by the regulator, exhibits very small temperature changes, the ordinary day and night variation being inappreciable. The regulator may be used for the largest furnaces by the addition of a relay, or relays, of suitable current-carrying capacity. It cannot be damaged by failure of the power supply, and returns to normal operation when the supply is restored.