Abstract: A linear motion encoder which employs a potentiometer type motion detector. It includes an elongated rectangular or cylindrical housing in which are one or two rails for guiding an insulated block to move lengthwise of the housing while being moved by a shaft connected to the block via an anti-backlash spring means. The block carries wiper contacts which respectively contact a fixed resistance element and a fixed commutator strip mounted on one wall of the housing or on the rail. An analog to digital circuit may be disposed in the housing to apply a reference voltage to opposite ends of the resistance strip. The circuit takes a linearly variable analog voltage from the commutator strip when the block moves lengthwise inside the housing driven by the axially moving shaft.