Original plate illustrating Michael Faraday's electric motor.
Credit: Quarterly Journal of Science, Literature and the Arts, 1821, volume XII

 

The Faraday Motor
Nov 12, 2010

Galaxies have been likened to "homopolar motors" invented by Michael Faraday. What are they and how do they work?

It has been argued that galaxies might behave like a device invented by Michael Faraday, the homopolar motor. A homopolar motor is driven by magnetic fields induced in a circular, rigid conductive metal plate. The metal plate is placed between the poles of an electromagnet, causing it to spin at a steady rate proportional to the input current. The meter attached to the wall in most backyards that determines monthly electric bills is a homopolar motor.

The homopolar motor owes its existence to the discovery of electromagnetism in 1820 by Hans Christian Ørsted, although that line extends much farther back in time. It was a so-called "voltaic pile," one of the earliest batteries, that opened Ørsted's eyes to the relationship between electricity and magnetism.

In 1800, Alessandro Giuseppe Antonio Anastasio Volta invented an apparatus made up of discs that included copper, zinc, and cardboard that was impregnated with a salt solution. He alternately stacked the discs and attached conductive wires to the top and bottom of the stack. When the two wires were connected in a circuit, an electric current flowed through the voltaic pile.

Ørsted noticed that a magnetized compass needle was deflected from its north-south orientation whenever the compass came close to the current flow. It was his observation that electric current and magnetism were related, coupled with André-Marie Ampère's mathematical analysis of said "electromagnetic" effect, that ultimately led to Faraday's experiment.

Faraday built a device out of a wire, a battery, and a mercury bath. The wire hung down loosely from a hook so that it made contact with the mercury. A permanent bar magnet stood upright in the mercury pool, which was connected to the negative battery terminal. The positive battery terminal was connected to the hook holding the wire. When current ran through the circuit going from positive to negative, the wire's magnetic field interacted with the bar magnet, causing a circular magnetic field around the wire, which began to spin around the bar magnet.

A force that is at right angles to both the direction in which a charged particle is moving and the direction of the applied field is called the "Lorentz force," and is an integral component to the operation of a homopolar motor.

The Dutch physicist Hendrik Lorentz found that the velocity and charge of a particle, as well as the strength of a magnetic field, influences the particle's direction of travel. When current flows through the homopolar motor's conductive disc in a magnetic field, a force acts on the charges in the conductor, producing torque that causes the disc to spin around its pivot point.

As retired Professor of Electrical Engineering Don Scott wrote:

"In 1986, Nobel laureate Hannes Alfven postulated both an electrical galactic model and an electric solar model. Recently physicist Wal Thornhill has pointed out that Alfven's circuits are really scaled up versions of the familiar homopolar motor that serves as the watt-hour meter on each of our homes. The simple application of the Lorentz force equation ('crossing' the direction, v, of the current into the direction, B, of the magnetic field) yields a rotational force. Not only does this effect explain the mysterious tangential velocities of the outer stars in galaxies, but also (in scaled down version) the observed fact that our Sun rotates faster at its equator than at higher (solar) latitudes."

Stephen Smith