Abstract: First and second tubes (16, 18) extend between first and second housings (12, 14). A magnet assembly (52) is slidably received in the first tube (16) which is formed of non-magnetic material. A timing belt (38) is attached to one side of the magnet assembly (52) and extends through tube (16) to the first housing (12) around timing pulleys (24, 26) through second tube (18) into second housing (14), around pulleys (28, 30) back into the first tube (16) and is attached to the other end of magnet assembly (52). A carriage (20) is slidably received on the first tube (16) and mounts thereon a second magnet (70) magnetically coupled to the magnet assembly (52). A wheel (74) or a block (92) having a bore therethrough rides on the second tube (18) to maintain the selected orientation of the carriage. First and second proximity switches (76, 78) are mounted in the housings adjacent the ends of the second tube (18) which respond to a sensor element (80, 81) attached to the timing belt (38).