Abstract: An apparatus for continuously adjusting the corona discharge currrent of a pair of electrodes exposed to an atmosphere of gaseous impurities, particularly halogen. The device consists of a power supply to cause a corona current to flow in a pair of electrodes in series with a summing resistor. Any change in the gaseous impurities which causes a change in the corona current is sampled in the resistor, detected, delayed, amplified and fed back in proper phase to the control element of the power supply to cause the corona current to remain constant. During the finite delay time the change in corona current causes an audible alarm to sound until the corona current is restored to the new level of impurity concentration. The corrective action is continuous for any level of impurity concentration.