Faraday's Electromagnetic Induction (1831): Birth of the Dynamo

Electromagnetic induction is the production of an electric current by a changing magnetic field. Michael Faraday first demonstrated it on 29 August 1831 in his basement laboratory at the Royal Institution in London, and within weeks he had built the first electric generator. His discovery underlies the dynamos, transformers and power stations that supply almost all of the world’s electricity.

Background: from electricity to magnetism and back

In 1820 the Danish physicist Hans Christian Ørsted showed that an electric current deflects a compass needle, proving that electricity can produce magnetism. The discovery caused a sensation. André-Marie Ampère in Paris quickly worked out the forces between currents, and in 1821 Faraday made a wire carrying a current rotate continuously around a magnet, the first electric motor in principle.

If electricity could make magnetism, many scientists reasoned, magnetism should be able to make electricity. Faraday noted the idea as a goal in his notebook in 1822, and others, including Ampère and François Arago, came close to the effect without recognising it. The difficulty was that a steady magnet next to a steady wire produces nothing. The key, which nobody had yet grasped, was change.

Faraday himself was an unlikely scientist. Born in 1791 in Newington Butts, south London, the son of a blacksmith, he left school early and was apprenticed to a bookbinder, educating himself by reading the books he bound. In 1813 the chemist Humphry Davy took him on as an assistant at the Royal Institution, where he spent the rest of his working life.

The ring experiment of 29 August 1831

Faraday had a ring of soft iron made, about 6 inches (15 cm) across. He wound two separate coils of insulated copper wire around opposite halves of it, which he called sides A and B. Side B was connected by a long wire to a galvanometer, a sensitive current detector, placed some distance away. Side A was connected to a battery.

At the moment he completed the circuit to side A, the galvanometer needle on side B jumped and then settled back to zero. When he broke the battery circuit, the needle jumped again, in the opposite direction. A steady current in A produced nothing in B; only switching it on or off did. Faraday recorded the result in his laboratory diary, the notebook in which he numbered and described his experiments.

He had found what later physicists called mutual induction. The current in coil A magnetised the iron ring, and the growing or collapsing magnetism induced a brief current in coil B. In modern terms the ring was the first transformer.

Magnets, coils and the first generator

Faraday pursued the effect through the autumn. On 17 October 1831 he pushed a bar magnet into a hollow coil of wire connected to a galvanometer and saw a current flow as the magnet moved in, and a reversed current as it was pulled out. Motion alone, with no battery at all, generated electricity.

On 28 October he went further. Using a large horseshoe magnet belonging to the Royal Society, he turned a copper disc between its poles and collected current from sliding contacts touching the rim and the axle. As long as the disc spun, a steady current flowed. This device, the Faraday disc or homopolar generator, was the first machine to turn mechanical motion continuously into electrical energy.

Faraday presented his results to the Royal Society on 24 November 1831 as the first series of his “Experimental Researches in Electricity”, published in the Philosophical Transactions in 1832.

Explaining induction: lines of force

Faraday had almost no formal mathematics. Instead he pictured the space around magnets and currents as filled with “lines of force”, which could be made visible by sprinkling iron filings on paper over a magnet. He described induction as what happens when a conductor cuts across these lines, or when the number of lines passing through a circuit changes. The faster the change, the larger the current.

This idea, now summarised as Faraday’s law of induction, was later given precise mathematical form by James Clerk Maxwell, who built Faraday’s field concept into his theory of electromagnetism in the 1860s. In 1834 the Russian-German physicist Heinrich Lenz added the rule that an induced current always flows in a direction that opposes the change producing it.

Joseph Henry and the question of priority

In Albany, New York, the American teacher and physicist Joseph Henry had been building powerful electromagnets and observed induction effects at about the same time, possibly earlier. Busy with teaching, he did not publish until 1832, after news of Faraday’s work had crossed the Atlantic. Scientific credit for the discovery went to Faraday as the first to publish, while Henry is credited with the discovery of self-induction. Both men are honoured in units: the farad for capacitance and the henry for inductance.

Consequences: dynamos, transformers and electric power

Faraday’s disc produced only a feeble current, and he left practical applications to others. In 1832 the Parisian instrument maker Hippolyte Pixii built a hand-cranked generator that spun a magnet past coils, producing alternating current; a commutator was soon added to give direct current. Magneto generators powered early arc lamps in lighthouses, beginning at South Foreland in 1858 with apparatus Faraday himself advised on.

The breakthrough to large-scale power came in the 1860s and 1870s with self-excited dynamos, which used part of their own output to energise their magnets. Werner von Siemens, Charles Wheatstone and others announced the principle around 1866–1867, and Zénobe Gramme’s ring dynamo of the early 1870s was efficient enough for industry. By the 1880s central power stations were supplying electric light, and transformers working on Faraday’s principle made long-distance transmission of alternating current possible, a development at the heart of the war of the currents.

Myths and facts

Myth: Faraday promised a minister that electricity could one day be taxed. In the best-known version, the Chancellor William Gladstone asks what use electricity is, and Faraday replies that the government will soon be able to tax it. No source from Faraday’s lifetime records the exchange, and historians regard it as a later invention, however well it captures the gap between pure research and its eventual uses.

Myth: Faraday invented practical generators and transformers. Faraday discovered the principle and built laboratory demonstrations, but he deliberately left commercial development to others. The practical dynamo and the power transformer were the work of many engineers over the following half-century.

Fact: the original apparatus survives. The induction ring of 1831 is kept at the Royal Institution in London, where Faraday’s restored basement laboratory can also be seen.

Legacy

Faraday went on to discover the laws of electrolysis, introducing words such as electrode, anode, cathode and ion, and in 1845 found that a magnetic field can rotate the plane of polarised light, a first hint that light is electromagnetic. He founded the Royal Institution’s Christmas Lectures for young people, which still continue, and declined both a knighthood and the presidency of the Royal Society. He died in 1867. His portrait appeared on the Bank of England £20 note in the 1990s, a sign of his standing as one of the greatest experimental scientists.

Timeline

• 1820, July: Ørsted publishes the magnetic effect of an electric current.
• 1821, September: Faraday demonstrates electromagnetic rotation.
• 1831, 29 August: Faraday’s ring experiment reveals electromagnetic induction.
• 1831, 17 October: A moving magnet induces a current in a coil.
• 1831, 28 October: Faraday generates a continuous current with a rotating copper disc.
• 1831, 24 November: Faraday reads his first series of “Experimental Researches in Electricity” to the Royal Society.
• 1832: Hippolyte Pixii builds a magneto-electric generator; Joseph Henry publishes his findings.
• 1834: Lenz states the law on the direction of induced currents.

Why it matters

Faraday’s experiments of 1831 showed that motion and magnetism could be turned into electricity. Every power-station generator, transformer and induction motor works on the principle he found, so the electrified modern world, from street lighting to the national grid, rests on a few weeks of experiments with an iron ring and a copper disc.