This reaction timer was "set" by swinging the lever to the far left where it was held in place by a catch. The experimenter released the catch which simultaneously presented a visual stimulus to the subject through the round hole and started the pendulum swinging. When the subject pressed the response key, electromagnets mounted on the swinging pendulum instantaneously clamped it to a curved metal plate and stopped it. The distance that it moved along the calibrated scale indicated the elapsed time. It was manufactured by Stanford White, 300 Broadway, New York., and carries serial number 142.