Abstract: A test probe for monitoring the level of liquid in a storage container, especially of a flammable liquid, has a guide tube which is inserted through and secured in the threaded hole of a cover of the container. The probe is provided with a magnetic indicating system mounted in part on a bearing which surrounds the tube within the container. Magnetically coupled to the bearing and movable along and within the tube is an electrical contact carriage which makes contact with fingers provided on rolled-on metal tracks provided on non-conductive profiled strips located at least in part in the tube. The strips are reinforced by profiled rails which are located in the tube.