Abstract: A motion sensor has a ball of electrically conductive material in an annular ball race around which the ball can travel in response to motion of the sensor and electrical contacts spaced apart from one another around the ball race for electrical contact with the ball during such travel. A circuit responsive to electrical contact of the ball with the contacts detects the travel of the ball and, thus, motion of the sensor and a device is provided for selectively retaining the ball in an inoperative position spaced from the contacts, whereby wear of the contacts by the ball is avoided while the sensor is in a correspondingly inoperative condition.