The problem of the precise control of the temperature of liquid flowing through a tank containing a heater is one which is important and familiar in the process-control industry. When the heater is electrical the thyristor is a natural choice for the main controller. A simple and novel method of triggering the thyristors is described in which the power dissipated in the heater is a linear function of the control signal. The method is compared with the conventional non-linear and linearized arrangements and its electronic implementation is described. A 0.75 kW model has been designed to demonstrate the principle. With the same basic circuitry but with more powerful gating circuits the arrangement could be used to control very large powers