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 O is subject to disturbances D 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.