To determine the value of and#8216;gand#8217; by timing the fall of a steel ball. The release unit consists of a plate 90 x 45 x 3mm carrying a pair of metal clips connected to 4mm sockets. The clips are opened, thus dropping the ball and breaking the contact between the clips. The trip unit is mounted on a base measuring 100 x 100 x 12mm and comprises of a trip plate, the end of which bears upon the blade of a micro switch and holds it in the closed position. When deflected downwards by the impact of the ball the trip plate releases the blade and opens the switch. The trip plate comes complete with a protective rubber strip to prevent the switch from damage should it be accidentally struck directly by the ball. Kit comes complete with steel balls of 16mm and 19mm diameter.