A length measuring apparatus comprises a measuring device for detecting the position of a marker along a measuring path. The measuring device and the marker are encapsulated in a hermetically sealed housing. A follow-up device is provided for moving the marker to a predetermined position of registration relative to a detectable point, which is disposed outside the housing and is movable along a path which is parallel to and laterally aligned with the measuring path. The follow-up device comprises a device for establishing a non-contacting link between the detectable point and the marker. To provide such an apparatus which has a relatively simple design and a low susceptibility to disturbance and can achieve a high accuracy of measurement. The apparatus comprises a strip that is trained around reversing end pulleys and has a course that extends along the measuring path. The marker is carried by the course. The strip is adapted to be driven by the follow-up device and is provided with a measuring scale and in the housing extends past a stationary reader for reading the measuring scale.