Abstract: A circuit arrangement for utilizing the "flame (or hot gas) rectification" effect in an air gap is provided in which the air gap is defined between electrodes one of which is connected to a source of A.C. and the other of which is connected to earth or other suitable potential. A non-linear resistance device is connected in series with the air gap and a voltage sensing is provided for detecting a voltage across the non-linear resistance device, the arrangement being such that when no flame or hot gas is present in the air gap the air gap and the non-linear resistance device are virtually open-circuited so that the A.C's alternated by filter means to zero volts D.C. with a small ripple, superimposed thereon at the input to the voltage sensing circuit, whereas when flame or hot gas is present in said air gap the non-linear resistance produces a negatively predominant signal which after filtering provides a negative D.C. voltage shift with a small percentage A.C.