A Magnetic Pickup consists of a permanent magnet, a pole-piece and a sensing coil all encapsulated in a cylindrical case. An object (target) of iron, steel, or other magnetic material, passing closely by its pole-piece causes distortion of the magnetic flux field passing through the sensing coil and pole piece, which in turn generates a signal voltage. The magnitude of the signal voltage depends on the relative size of the magnetic target, its speed of approach and how close it approaches. The polarity of the signal depends on whether the target is moving toward or away from the pole-piece. Magnetic Pickups are most frequently used to sense passing teeth on a gear, sprocket, or timing belt wheel, to bolt-heads, key-ways, or other moving machine mounted targets.