This type of switch, of course, only works with granular solids and liquids that are electrically conductive (e.g. potable or dirty water, acids, caustics, food liquids, coal, metal powders) and not with nonconducting materials (e.g. ultra-pure water, oils, ceramic powders).
Line voltage (120 VAC) energizes the primary coil, sending a magnetic field through the laminated ferrous6 core of the relay. This magnetic field easily passes through the centre of the secondary coil when the secondary circuit is open. Thus completing the magnetic “circuit” in the core. With the magnetic circuit thus completed, the armature will not be attracted to the core. However, when a circuit is completed by liquid level rising to contact both probes, the secondary coil’s resulting current “bucks” the magnetic flux through its centre, causing more magnetic flux to bypass to the end poles where it attracts the ferrous armature toward the core frame.
When the level increases the eventually the armature gets attracted to the secondary coil, closing the switch attached to the armature results in actuating the indicator.