What is a homopolar motor?
The name means that the electrical polarity of the motor never changes. The axis of the rotating part of the motor is parallel to the magnetic field from the permanent magnet. Lorentz forces is what makes the motor turn, a conductor conducting current through a magnetic field will respond to a external magnetic field and will gain velocity in what direction the fields happen to be aligned. The hand rule applies here.
This motor is restricted to a single turn coil which limits it to small voltages around a couple of Volts and it has a low torque.
The homopolar motor will also work as a generator if we remove the battery and apply the motion to the rotating disc. It is only capable of supplying a couple of Volts but at very high currents, generators can be put in series for higher voltages. Very large homopolar generators are used for high current short circuit testing.
Warning: this motor operates at high speeds up to 6000 RPM, the screw can at any time loose contact and fly through the air.
Michael Faraday demonstrated this motor concept in 1821 at the Royal Institution in London. The homopolar motor is the first electrical motor to be built. His construction was a large and crude concept that he himself describes as “This apparatus may be much reduced in size, and made very much more delicate and sensible.”
The original article in Quarterly Journal of Science:
How can you perform this experiment yourself?
You will need the following four items to make this experiment.
- 1x 1.5 Volt battery. I use a AA battery
- 1x iron wood screw.
- 1x piece of copper wire.
- 1x round neodymium disc magnet.
Follow these four steps to make it work.
- Place the head of the screw in the centre of the magnet.
- The screw must be touching the positive terminal of the battery. Sides of the battery are insulated.
- Hold the wire with your finger against the negative pole terminal of the battery.
- Gently let the other end of the wire touch the side of magnet, completing the circuit.
- Watch as the magnet and screw starts to spin faster and faster.
A quick experiment that can be made from common household items. It is a simple practical demonstration of the basic electro magnetic theory, that would help many understand the force around a wire easier.
Published August 4, 2011. Updated October 31, 2021.