Aim 712-M8, 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.
M8 Thread, 712 plug