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? In simple terms, a motor is a device that converts one form of energy into mechanical energy. Motors can draw from different energy sources. Combustion engines use thermal energy from burning fuel, while electric motors generate mechanical power from electrical energy.
Electric motors can be classified in many ways, depending on their design and how they operate. One of the main distinctions is the type of electric current they use. Some motors run on direct current (DC), while others operate on alternating current (AC).
The first working electric motor powered by direct current was built by Michael Faraday and demonstrated in London in 1821. It was known as the 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 valued experimentation as a key path to discovery but also emphasized the importance of theory. Faraday studied how electric current affects metallic conductors and electrolytes, formulating the laws of electrolysis that bear his name. He also discovered electromagnetic induction and self-induction, laying the groundwork for modern electrodynamics. In addition, he built the first electric generator and the first direct current motor.
Today, homopolar motors are mostly of historical interest, having been replaced by more advanced designs such as the commutator-type DC motor. Still, their simplicity makes them ideal for home or classroom experiments. Building one yourself, even though it produces little power, is a rewarding experience and a great hands-on way to apply what you’ve learned in physics class.
To make a working model of a homopolar motor, you’ll need a standard 1.5 V Leclanché cell. I recommend using an R14 cell, also known as a C-size battery (Fig. 1A). You’ll also need a magnet, preferably a neodymium type, with a diameter of about 29 mm (roughly 1.14 in) (Fig. 1B).
Warning: Large neodymium magnets can be dangerous. Handle them with great care. If a finger gets caught between two magnets, it may be extremely difficult and painful to remove.
Author’s note: Make sure neither the battery nor the wire overheats during operation. Excessive heat can cause burns or, in the worst case, make the battery explode.
For the experiment to work correctly, the magnet’s surface must be electrically conductive.
The key component is the rotating part, called the rotor (Fig. 1C). It is made from a piece of 0.5 mm (0.02 in) thick uninsulated copper wire. Figure 2 shows the rotor design in more detail. The upper part of the wire is bent into a small loop (A), which serves as the axis of rotation. The lower part forms a ring (B) that is slightly wider than the magnet’s diameter by a few millimeters (about 0.1–0.2 in). This design provides good stability, and the rotor should 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