The free fall apparatus comprises two units, each of which is mounted on a plastic base, which can be clamped on a 13mm diameter retort rod. Connections are brought out to 4mm sockets. The solenoid unit has a metal core, which ensures prompt release of the ball on de-energising. The gate switch unit comprises a hinged steel plate, which is held against an adjustable contact by means of a small permanent magnet, so that, by proper adjustment, the plates fall away promptly at the impact of the falling ball.