Abstract: In a device for monitoring the liquid level in a tank, utilizing a reed switch projecting into the tank in an internally cylindrical hollow rod and a float provided with a permanent magnet and guided on the hollow rod, the reed switch is constructed as an independent exchangeable part introducible into the hollow rod with an accurate fit and with a small assembly outlay. The slide-in part has three radially resilient abutment zones distributed uniformly around the circumference. In order to accurately introduce the slide-in part into the hollow rod, with a minimum of force, three axial guide grooves associated with the resilient abutment zones are formed radially outwards in the inner envelope surface of the hollow rod. In order to achieve the final accurate fit, the slide-in part, introduced completely into the hollow rod, is rotated until the resilient abutment zones are located in regions between the guide grooves of the hollow rod.