1. When electrode E1 is not in contact with the conductive liquid as shown in figure 2, the electrical circuit is open, and no current flows between electrodes E1 and E3. Consequently relay X does not operate and the contact remains at the b side.
2. When electrode E1 is in contact with the conductive liquid as shown in figure 3, the circuit closes due to the conductive fluid completing the circuit between E1 and E3. Relay X operates and switches to the a side.
By connecting the relay contacts to a contactor, the pump can be turned ON and OFF.
However in practice, with only two electrodes, ripples on the surface of the liquid cause the relay to switch rapidly. This problem can be solved by forming a self-holding circuit. (The configuration shown in figures 2 and 3 can be used as water level alarms.)