KOBOLD limit switches of model NES are used for level monitoring and pump control of conductive liquids. The design without any moving parts allows service with critical media with, for example, solid content, negligible density or high viscosity. The instruments operate on the conductive principle. A low a.c. voltage is applied between the conductive wall of the tank or the earth electrode (longest electrode) and a switching point electrode. If the conductive medium touches the electrodes, a negligible alternating current flows across the electrodes and the conductive medium to the electrode relay. The relay amplifies the alternating current and operates an relay or a pump controller. An electrode relay of type NE-104 is required per switch point for signalling. For min./max. control two switching point electrodes must be connected to the relay.