The ball can be released from four different heights, 0.5m, 1m, 1.5m and 2m. Place the ball between the contacts. Slide the rod to the left and tighten set screw to hold ball in place. Tap the contact pad and reset the timer, in that order. Loosen set screw to release ball. The timer will start. The timer stops when the ball strikes the pad. Move ball release to a new position and repeat. Gives g to within 5%.