Abstract: A method and apparatus for measuring the presence of trace amounts of a halogen gas in ambient air by changes in conductivity of a negative corona in an electrical sensing gap due to the presence therein of such electro-negative gas molecules, utilizes a digital feedback loop sensitive to gap corona current to adjust the high voltage producing the corona discharge to obtain the optimum corona current for best sensitivity irrespective of the concentration of the halogen contaminant present. The presence of the halogen gas contaminant in the air is indicated by a bi-level audible alarm consisting of a periodically interrupted tone the frequency of which goes above or below a base line value to indicate small variations in impurity concentration above or below the base line concentration, and the tone interruption rate of which, at the same time, increases for large impurity concentrations.