[en] An ionisation smoke detector is described which, since a source of less than 1 μ Ci is used, escapes the legislative restraints normally imposed on such detectors. The system comprises a detector casing, outer, inner and intermediate electrodes defining inner and outer interconnected ionisation chambers and a 0.8 μ Ci ²⁴¹Am source whose position in the inner chamber is adjustable. The volume of the inner chamber and the location of the source being such that when the inner chamber is ionisation-saturated, the outer chamber is not so saturated. (UK)