Abstract: Method and apparatus for sensing solid particles in a mass of gas, for instance for detecting smoke in air and thus acting as a fire alarm. The gas is ionized and the remaining ionization is then detected after a time delay during which the smoke particles capture a proportion of the created ions. The time delay may be created either by physically separating the ionizing source and the detector and causing the mass to move from one to the other, or by operating source and detector intermittently with a time gap between each ionization and the subsequent detection.