Aim 719 Hall Speed Pick up Sensor (1m cable). The output varies when it meets a magnetic field. The three-wire sensor produces a digital signal.
The sensor consists a thin piece of metal what passes a continuous current through it. When the sensor is placed in a magnetic field the electrons move to one side of the metal and protons move to the other side of the plate. Then you will be able to get a voltage.
The sensor only produces around seven volts. When the electrons move to one side a potential difference is produced between the two sides of the metal.