The heater is driven from a voltage-controlled power supply, the voltage V1 being derived from a potentiometer. The output temperature, 0 degree, is subject to disturbances, 0D, 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.