The Off-Key Bard
History

This Day in History — Faraday Demonstrates the First Electric Generator

On this day in 1831

On October 28, 1831, Michael Faraday spun a 12-inch copper disc between magnet poles and produced a steady electric current from motion alone.

The Off-Key Bard enters a scientific laboratory in October 1831. There are no electrical outlets, no glowing light bulbs, and no machines humming with power from a distant generating station. Instead, a scientist is working with an enormous magnet, a copper disc and a sensitive instrument that can detect the faintest electric current. He begins turning the disc, and the instrument responds.

"For the first time, mechanical motion is producing a continuous electrical current, and the future of electric power generation is beginning to take shape."

On October 28, 1831, the British scientist Michael Faraday carried out an experiment in which a copper disc rotating between the poles of a strong magnet produced a steady electric current. The apparatus, now known as the Faraday disc and described as a homopolar or unipolar generator, is generally regarded as the first electromagnetic generator. It came two months after his discovery of electromagnetic induction, and it answered the main limitation of that discovery: the current from the induction experiment had been only a brief pulse.

Electricity Before the Generator

In the early 19th century, producing a steady flow of electricity was hard. In 1800 Alessandro Volta described the voltaic pile, a stack of alternating zinc and copper discs separated by brine-soaked cloth or cardboard, which was the first battery able to supply a steady current. It let scientists build circuits and run chemical experiments with electricity, such as splitting water. But a battery runs on chemicals that are used up, and it was not a practical way to produce large amounts of power.

The next step came from the link between electricity and magnetism. In 1820 Hans Christian Ørsted showed that an electric current deflects a compass needle. André-Marie Ampère quickly repeated and extended the experiment and concluded that electric current produces magnetism, a result that contradicted the prevailing view that the two were separate.

The Bookbinder Who Became a Physicist

Michael Faraday was born on September 22, 1791, at Newington Butts, then a village outside London in Surrey. His father, James, was a blacksmith. Faraday began as an errand boy for the bookseller George Riebau and then served a seven-year apprenticeship as a bookbinder, reading many of the books he bound. In 1812 he attended Humphry Davy's lectures at the Royal Institution and copied out his notes. Davy later hired him as an assistant, and Faraday started work at the Royal Institution in 1813.

His strength was designing experiments. In 1821 he built devices that produced electromagnetic rotation, including the homopolar motor, which became a foundation of electric motor technology. The reverse question stayed with him: if electricity could make something move, could motion and magnetism make electricity?

August 1831: A Pulse

The first answer came in the basement laboratory of the Royal Institution. In late August 1831 Faraday wound insulated copper coils on opposite sides of an iron ring. The Royal Institution dates his success to August 29, 1831. When current from a battery was switched on in one coil, a galvanometer connected to the other showed a brief current, and the needle moved again when the battery was disconnected. The effect was a pulse, not a steady flow. The ring is now in the Faraday Museum at the Royal Institution.

Within weeks he was producing current directly from a magnet, moving a magnet in and out of a cylindrical coil of wire. By mid-October 1831 he had current from motion alone, but it still came in pulses, and it needed the magnet to keep moving in and out.

October 28, 1831: The Disc

Faraday's own account says his numerous experiments with the large magnet of the Royal Society were made at Mr. Christie's house in Woolwich, where the magnet was kept. His diary entry for October 28, 1831 records the disc experiment. The disc was a copper plate twelve inches across and about one fifth of an inch thick, mounted on a brass axle. Its edge was amalgamated with mercury to make a good but movable contact, and the same was done to a part around the axle. Faraday used the magnet's large poles, with two small magnets, six or seven inches long, placed against them to concentrate the field.

He connected one galvanometer wire to the axle and held a collector against the edge of the disc between the poles. When the disc was turned the galvanometer needle moved, and when it stopped the needle returned to rest. Early trials were irregular, but more careful runs gave what Faraday called a permanent deflection of nearly 45 degrees, which he described as a permanent current of electricity produced by ordinary magnets.

The arrangement worked from a different direction than the earlier experiments. Instead of moving a magnet through a coil, Faraday moved a conductor through a magnetic field. The result was continuous for as long as the disc kept turning.

Why the Disc Works

Copper contains mobile electric charges. When the disc spins through the magnetic field, the magnetic part of the Lorentz force acts on those moving charges and pushes them toward the center or the rim, which sets up a voltage between the axle and the edge. With contacts on both, current can flow through an outside circuit.

The energy does not appear from nowhere. It comes from the work of turning the disc. As the generator delivers current, electromagnetic forces oppose the rotation, so the disc keeps turning only if something keeps pushing it. A generator converts mechanical energy into electrical energy.

The disc also explained an older puzzle. In 1824 François Arago had reported that a spinning copper disc could drag a nearby compass needle and that copper damped a needle's swing, an effect he called the magnetism of rotation. Faraday showed that relative motion between a magnet and a copper disc induces currents in the metal, and that those currents produce forces that oppose the motion. His disc was the experimental basis for explaining Arago's observations.

Small, Weak and Important

The disc was not a practical power source. It produced a small direct-current voltage. Current flowed backward in parts of the disc outside the magnetic field, which cancelled some of the output and heated the copper, and the single current path from axle to rim kept the voltage low. Later machines used coils of wire to get higher voltages.

The principle was what mattered. In 1832 the Paris instrument maker Hippolyte Pixii built an early alternating-current generator in which a hand-cranked magnet spun past an iron-core coil. At Ampère's suggestion he added a commutator, which turned the output into pulsing direct current. In the 1860s several inventors showed that a generator could use part of its own output to energize its magnets. Ányos Jedlik is credited with the principle of self-excitation, Wilde and Siemens demonstrated a self-exciting dynamo by 1866, and Siemens, Wheatstone and Varley announced their dynamo-electric machines in 1866 and 1867.

Practical use followed. Power-driven machines were used for electroplating in Birmingham by 1841, and a generator for electric light was installed at the North Foreland Lighthouse by 1858.

An Idea Bigger Than Its Machine

Faraday's induction work reached well beyond generators. The August ring is widely regarded as the first electrical transformer. His experiments also shaped the understanding of electric and magnetic fields, which James Clerk Maxwell later put into mathematical form, and Maxwell's model of Faraday's law of induction became one of the four Maxwell equations. The unit of capacitance, the farad, is named after Faraday.

Today most large-scale electricity is still generated by turning a generator. At a hydroelectric dam, water drives the turbine. At coal, gas and nuclear plants, heat makes steam that does it. Wind turbines turn a generator directly. Solar panels and batteries are exceptions, since they produce electricity without a rotating generator. The Royal Institution still keeps Faraday's iron ring, a piece of apparatus built from ordinary materials.

A copper disc began to turn,
And through the field, new currents stirred.
From motion came electric flow—
A spark of power the world would know.

Faraday's disc could not light a city or run a factory. What it showed was that electricity could be made from motion and magnetism, and later engineers built larger and more practical machines on that principle.

On October 28, 1831, Faraday was not designing a power grid. He was studying what happens when a conductor moves through a magnetic field. That relationship became part of the foundation of modern electrical power.

Sources

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