Current sensing relays are found on most aircraft types. A common use for them is verification of pitot/static heating systems.
When pitot/static devices have the heat turned on, one leg of the heater element is passed through the sensing coil of the relay. With current present, the relay energizes and turns off any associated lights or warnings.
When troubleshooting a heating system, the device heater coil can usually be verified by continuity checking from the relay itself.