This apparatus is used to determine G by measuring the time of fall of a steel ball through a pre- determined distance. The time taken being less than one second, a centi second/ milli second timer capable of measuring the time in fraction of seconds has to be used. Comprises two units each of which is mounted on a plastic which can be clamped on a 12 mm diameter retort rod. Connections are taken out to 4 mm sockets. The solenoid unit has a metal core which facilitates prompt release of the sphere on de-energizing. The gate switch comprises a hinged steel plate to hold against an adjustable contact by means of a small permanent magnet, the plate falls away promptly at the impact of falling steel ball. The apparatus is supplied with a solenoid unit, gate switch, Steel Ball 18 mm dia and a stand.