Homopolar Motor
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The following article was originally published in the journal Młody Technik (eng. Young Technician) (3/2014):
Homopolar Motor
What exactly is a motor? Broadly speaking, a motor is a device that converts another form of energy into mechanical energy. Motors can draw from various energy sources — combustion engines use thermal energy from fuel combustion, while electric motors generate mechanical power using electrical energy.
Electric motors can be classified in many ways based on their construction and operating principles. One of the primary distinctions is the type of electric current used: some motors run on direct current (DC), while others operate using alternating current (AC).
The first working electric motor powered by direct current was built by Michael Faraday and demonstrated in 1821 in London. This was the so-called homopolar or unipolar motor.
Michael Faraday (1791–1867) was a self-taught British physicist and chemist who became one of the most influential scientists in history. He highly valued experimentation as a path to discovery, while also giving due importance to theoretical understanding. Faraday investigated the effects of electric current on metallic conductors and electrolytes (he formulated the laws of electrolysis, which bear his name). He also discovered and described electromagnetic induction and self-induction, laying the foundation for modern electrodynamics. Additionally, he built the first electric generator and the direct current motor.
Today, homopolar motors have primarily historical importance, having been supplanted by more advanced designs like the commutator-type DC motor. Still, their simplicity makes them perfect for building at home or in a classroom. Constructing such a device with your own hands — despite its limited power — can be deeply rewarding and offers a hands-on way to apply knowledge from school physics lessons.
To build a working model of a homopolar motor, you’ll need a standard 1.5V Leclanché cell. I recommend using an R14 cell or — in other word — C size battery (Fig. 1A). You’ll also need a magnet — preferably a neodymium one — with a diameter of around 29 mm (about 1.14 in) (Fig. 1B).
Warning: Large neodymium magnets can be dangerous. Handle them with extreme care. If a finger gets caught between two magnets, removing it may be extremely difficult and painful.
Author’s note: Be careful that neither the battery nor the wire gets too hot during operation, as this could result in burns or — even worse — cause the battery to explode.
For the experiment to work properly, the magnet’s surface must be electrically conductive.
The most important component is the rotating part — the rotor (Fig. 1C) — which is made from a piece of 0.5 mm (0.02 in) thick uninsulated copper wire. Fig. 2 shows the rotor design in more detail. The upper part of the wire is bent into a small loop A, which acts as the axis of rotation. The lower part of the rotor forms a ring B, slightly wider than the magnet’s diameter — by a few millimeters (around 0.1–0.2 in). This construction provides good stability. The rotor must be as well balanced as possible.
When the motor is running, the rotor’s loop will rest on the battery’s positive terminal (Fig. 3), allowing it to spin freely with minimal friction.
Place the battery on top of the magnet and ensure a good electrical connection between the battery’s negative terminal and the magnet’s surface. Then place the rotor on top, adjusting its length so that the copper ring gently touches the side of the magnet (Fig. 4), completing the electrical circuit.
If the motor doesn’t start spinning immediately, give the rotor a gentle push. A well-built homopolar motor can reach quite high rotation speeds and will continue running until the battery is drained. You can turn it off by simply removing the rotor.
Explanation
But how exactly does this simple motor work? Fig. 5 shows a diagram of its key components and interactions.
An electric current I flows in a closed loop through the conductive rotor and the magnet, moving from the positive terminal to the negative one. The permanent magnet creates a magnetic field B. The magnetic field lines run from the magnet’s north pole (N) to its south pole (S). It’s known that a conductor of length l, carrying a current I and placed in a magnetic field of induction B, is subject to an electromagnetic force F, defined by the equation (where α is the angle between the current’s direction and the magnetic field lines):
This force acts perpendicular to both the current direction and the magnetic field lines. Its direction is determined by the left-hand rule (used for motors), as illustrated in the diagram. This force causes the rotor to spin. By reversing either the polarity of the magnetic field or the voltage source, the motor can spin in the opposite direction.
It’s also worth noting that, like many electrical machines, this motor is reversible — it can function as a generator. Devices like this are still occasionally used in laboratory settings to generate extremely high electric currents.
References:
- Lancaster D., Understanding Faraday's Disk, Tech Musings 1997,
- Grotowski M., Michał Faraday: jego życie i dzieło 1791-1867, Księgarnia Św. Wojciecha 1928
All photographs and illustrations were created by the author.
Addendum
The effect of this experiment can be seen in the following video:
Marek Ples