This invention is a smoke detector for giving a warning that a fire is imminent. The detector has an ionisation chamber with a central electrode which is covered by insulation except at a restricted area closely spaced from a measuring electrode, a second electrode being constituted by a perforated wall of the ionisation chamber. The detector is very sensitive to the presence of smoke, because of the concentration of the electric field in the area between the measuring eleotrode and the exposed part of the first electrode.
