This type of switch only works with granular solids and liquids that are electrically conductive (dirty water, acids, caustics, food liquids, coal, metal powders) and not with nonconducting materials (ultra-pure water, oils, ceramic powders).
A special design for conductive level switches is shown below, using a special transformer/relay to generate an isolated AC probe voltage and sense the presence of liquid:
120 VAC line voltage energizes the primary coil, sending a magnetic field through the laminated iron core of the relay. This magnetic field easily passes through the center of the secondary coil when the secondary circuit is open (no liquid closing the probe circuit), thus completing the magnetic “circuit” in the core. The armature will not be attracted to the core even if the magnetic circuit is completed. 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 center, causing the more magnetic flux to bypass to the end poles where it attracts the iron armature toward the core frame. This physical attraction actuates switch contacts which then signal the presence of liquid level at the probes.
The following pair of illustrations shows the two conditions of this level switch, with the magnetic lines of flux, highlighted as dashed lines through the core:
The conductive level switch has the “transformer” design provides electrical isolation between the probes and the energizing (120 VAC) circuit. It also enables a wide range of detection voltages to be generated for the probes just by altering the number of wires “turns” in the secondary coil.
More modern variations on the same design theme use much lower AC voltages to energize the probes, employing sensitive semiconductor amplifier circuits to detect probe current and signal liquid level.
Instrumentation Basics 