Abstract: Two cup-shaped electrodes respectively defining outer walls of a closed and an open ionization chamber and an insulator wall sandwiched therebetween defining an interior wall of both chambers and carrying both an electrode common to both chambers and at least one circuit element of a sensing circuit responsive to the relative impedances of the two chambers for indicating the presence of smoke in the open chamber. Minimizing leakage current-increasing factors as moisture, dust, etc., the body of the circuit element is protectively mounted to the insulator wall within the closed chamber by means of a plurality of leads extending through mating holes in the insulator wall with a header surface of the body pressed against the wall. One of the leads is folded around the edge of a hole in the wall wherein the common electrode is mounted and is squeezed between the insulator wall and a part of the common electrode to make electrical contact therewith.