Length measuring apparatus

In a length measuring apparatus for indicating the position of a detectable point on a member movable to move said detectable point along a first linear path, which apparatus comprises

• a hermetically sealed housing,
• a marker mounted within said housing to be movable along a predetermined linear measuring path having opposite ends,
• said housing being adapted to be mounted so that said measuring path is parallel to and laterally aligned with said first path,
• follow-up means for moving said marker along said measuring path to a predetermined position of registration relative to said detectable point, which follow-up means comprises means for establishing a non-contacting link between said detectable point and said marker, and
• measuring means mounted within said housing for indicating the position of said marker along said measuring path,
• the improvement comprising
• two rotatably mounted rollers disposed in said housing beyond opposite ends of said measuring path and a strip extending between and around said rollers in contact therewith and having a tensioned course which extends along said measuring path and carries said marker,
• said follow-up means comprises drive means operatively connected to said strip and operable to move said marker by means of said strip along said measuring path, and means for detecting the extent and direction of any deviation of said marker from said predetermined position of registration relative to said detectable point, the detecting means transmitting a control signal to the drive means to move the strip and keep the marker at the predetermined position of registration relative to the detectable point.