Sensors

Magnetic sensors detect a varying field in an airgap using a Hall effect or magnetoresistance probe which delivers a voltage depending on B. Magnetic position and speed sensors used in brushless motors and automobile system controls offer reliable noncontact sensing in a hostile environment involving dirt, vibration and high temperatures. 
The point to emphasize here is that a permanent magnet usually forms part of the circuit; it provides a noiseless magnetic field with no expenditure of energy. For example, a variable reluctance sensor with a magnet and a Hall probe may be used to detect the rotation of a toothed wheel.