The block diagram of the control of an electric heating system. The heater is driven from a voltage-controlled power supply, the voltage V1 being derived from a potientiometer. The output temperature, θO, is subject to disturbances, θD, because of changes in the ambient temperature. It is proposed to apply 'disturbance feedback control' to the system by the inclusion of a transducer that measures the external temperature and feeds a signal back to the input via a proportional controller of gain H. Determine the required value of H to eliminate the effect of the disturbance.