Faraday's Induction: From Magnets to Electricity

On August 29, 1831, Michael Faraday connected a battery to a coil of wire wound on an iron ring and watched a needle jump in a second coil that had no battery at all. The needle moved only for an instant, when the circuit was made or broken, and then settled back to zero. That brief twitch was electromagnetic induction: a changing magnetic field producing an electric current. This article explains what Faraday did, what the physics says, and why nearly every power station on Earth depends on it.

The electrical world Faraday inherited

By the 1820s, electricity and magnetism were both well known and, as far as anyone could tell, unrelated. Alessandro Volta had described his battery, the voltaic pile, in a letter to the Royal Society published in 1800, giving experimenters a steady current instead of the sudden discharge of a static machine. Humphry Davy put the pile to work. In 1807, he used electrolysis to isolate potassium, and later that year sodium (Wikipedia, Humphry Davy). For the chemistry behind Volta's device, see Volta's battery and electrochemistry.

Then came 1820. Hans Christian Ørsted showed that a current in a wire deflects a compass needle, the first evidence that electricity could produce magnetic effects. Ørsted's discovery set off a rush of work, and André-Marie Ampère quickly worked out the mathematics of the forces between current-carrying wires.

That left an obvious question, one that Faraday's contemporaries asked repeatedly. If a current can make a magnet move, can a magnet make a current flow? Ørsted's result looked like a one-way street, and for eleven years nobody found the way back.

A bookbinder's apprentice at the Royal Institution

Faraday was born on September 22, 1791, in Newington Butts, then in Surrey and now part of south London. His father, James, had been a blacksmith's apprentice in Westmorland before the family moved to the capital. At 14, Michael began a seven-year apprenticeship with George Riebau, a bookbinder and bookseller, and read many of the books that passed through the shop (Wikipedia, Michael Faraday).

In 1812, at age 20, he attended lectures by Humphry Davy at the Royal Institution and sent Davy a bound, 300-page book of his notes. When Davy needed an assistant after a laboratory accident, Faraday was appointed Chemical Assistant on March 1, 1813. In 1825, he became Director of the Laboratory of the Royal Institution, and in 1833 the first Fullerian Professor of Chemistry.

Faraday's education was thin in mathematics. Sources describe his skills as limited to the simplest algebra, with no trigonometry. Yet James Clerk Maxwell later wrote that Faraday's use of lines of force showed him "to have been in reality a mathematician of a very high order." His mathematics was geometric and physical, done in pictures of space instead of symbols. A fuller portrait is in Michael Faraday: the bookbinder who rewired the world.

The 1821 experiment that came before

Faraday had already built a working electromagnetic device a decade earlier. In 1821, shortly after Ørsted's discovery, he built two devices he called electromagnetic rotation. One of them, now called a homopolar motor, made a wire dipped in mercury circle a magnet when a battery pushed current through it. It was the first demonstration of electrical energy turning into continuous motion, and it matters here because Faraday spent the following years trying to make the process run in reverse. How that early motor evolved is told in the electric motor.

What happened on August 29, 1831

Faraday's earlier attempts tended to use steady magnetic fields: a magnet held near a wire, a constant current in a neighboring coil. Nothing happened, because nothing was changing.

In August 1831, he made a new apparatus in his basement laboratory. According to the Royal Institution, which holds the original, it is an iron ring 32 millimeters high and 170 millimeters across, wound with copper wire insulated with cotton. Winding two coils on opposite sides of the ring and insulating them was slow work, probably about ten working days. He succeeded with it on August 29, 1831.

One coil, which Faraday called A, was connected to a battery. The other, B, was connected to a galvanometer, a sensitive instrument that deflects when current flows. When he connected the battery, the galvanometer needle kicked and returned to zero. When he disconnected it, the needle kicked the other way. In between, with a steady current flowing in A, nothing registered in B. In the published paper, Faraday wrote of "the first induced wave of electricity."

Think of it as a doorbell for change. A steady current in coil A produces a steady magnetic field in the iron ring, and the second coil sits through that unchanging field without noticing. Only the moments when the field grows or collapses ring the bell.

The magnet-and-coil experiments

Within two months, Faraday found induction without any battery. He slid a bar magnet into and out of a coil of wire, and the galvanometer deflected, one way going in and the opposite way coming out. A magnet at rest inside the coil produced no current, however strong the magnet. His laboratory diary for October 28, 1831, records him "making many experiments with the great magnet of the Royal Society."

The first paper on his results is marked as read to the Royal Society on November 24, 1831, and appeared in Philosophical Transactions in 1832 as the First Series of his Experimental Researches in Electricity. Joseph Henry in the United States made similar observations independently, but Faraday was the first to publish.

Date	Event
1800	Volta's pile described in a letter to the Royal Society
1807	Davy isolates potassium and sodium by electrolysis
1820	Ørsted shows a current deflects a compass needle
1821	Faraday builds his electromagnetic rotation devices
August 29, 1831	Faraday's iron-ring experiment shows induction between two coils
October 28, 1831	Diary entry records experiments with the Royal Society's great magnet
November 24, 1831	First paper read to the Royal Society
1832	Joseph Henry makes similar observations independently
1834	Emil Lenz states the rule for the direction of induced current
1845	Franz Neumann gives induction a mathematical formulation
1864	Maxwell reads his electromagnetic field theory to the Royal Society

The first generator: a spinning copper disk

Faraday's most direct step toward power generation was a copper disk spun between the poles of a magnet, with one electrical contact at the center and another at the rim. A voltage appeared between the two contacts, and as long as the disk turned, current flowed. It is called a homopolar generator, and it was Faraday's first generator. The EIA traces the electromagnetic generators used today back to it.

The disk was not practical, though. Current circulated backward in regions outside the magnetic field, wasting energy and heating the copper, so it delivered little power. Later designs fixed the problem. The deeper point is that for the first time, mechanical motion had been turned directly into electrical current, with no chemistry involved.

Faraday's law of induction, with real numbers

Faraday described his results in words and in pictures of magnetic lines of force. The compact modern statement came later. The law says that the electromotive force (emf, the voltage driving the current) around a closed circuit equals the negative rate of change of magnetic flux through it. For a coil of N turns, multiply by N:

emf = −N × (change in flux ÷ change in time)

Magnetic flux is the amount of magnetic field passing through a surface. For a uniform field B crossing a flat loop of area A at a right angle, the flux is B × A, measured in webers (tesla times square meter).

A worked example makes it concrete. Take a coil of 100 turns, each 10 centimeters on a side, so A is 0.01 square meters. Move it into a field of 0.5 tesla in 0.1 seconds.

• Flux change per turn: 0.5 T × 0.01 m² = 0.005 Wb
• Rate of change: 0.005 Wb ÷ 0.1 s = 0.05 V per turn
• Total for 100 turns: 100 × 0.05 V = 5 V

Slow the same motion to one second, and the voltage drops to 0.5 volts. The size of the effect depends on how fast the flux changes, not on how strong the field is at any given moment. That is why a stationary magnet, however strong, does nothing.

There are three ways to change flux through a loop. You can change the field strength, as Faraday did by switching the battery on and off. You can change the area, by squeezing or stretching the loop. Or you can change the angle between the loop and the field, which is exactly what a rotating generator coil does.

Way to change flux	Example
Change B (field strength)	Switching current in coil A on and off; the primary of a transformer
Change A (loop area)	Stretching a loop of wire in a field
Change the angle	Spinning a coil in a magnet's field, as in a generator
Move the magnet or coil	Pushing a bar magnet through a loop

Who wrote the equation

Faraday did not write down the law as an equation. His notebooks and papers are mostly in words and sketches. Franz Ernst Neumann formulated the laws of induction mathematically in the mid-1840s, and Maxwell absorbed Faraday's insights into his broader electromagnetic theory in the 1860s (Wikipedia, Faraday's law of induction). The version in physics textbooks is a product of several people, built on Faraday's experiments.

Lenz's law: why the sign is negative

The minus sign in the law has a physical meaning. In 1834, Emil Lenz formulated the rule that gives the direction of the induced current: the current flows so that its own magnetic field opposes the change that produced it. Push a magnet's north pole toward a coil, and the coil's current creates a field that pushes back on the magnet.

This is not an arbitrary convention. It is conservation of energy at work. If the induced current aided the change that caused it, pushing the magnet in slightly would make the current stronger, which would pull the magnet in harder, which would make the current stronger still, and you would have a perpetual motion machine producing power from nothing. Because induced currents resist, you have to do mechanical work to produce electrical energy. The effort you feel pushing a magnet into a coil connected to a light bulb is the energy being converted.

A well-known classroom demonstration shows the same effect without any circuit. A strong magnet dropped down a copper tube falls noticeably slowly, because the moving magnet induces circulating currents (eddy currents) in the copper, and those currents create fields that resist the fall. The kinetic energy that would have gone into speed goes into heating the tube instead.

How generators turn Faraday's law into electricity

A generator is Faraday's experiment made continuous. A coil rotates in a magnetic field. The angle between the coil and the field keeps changing, so the flux keeps changing, so a voltage is induced.

For a coil of N turns and area A rotating at angular speed ω in a field B, the flux is B × A × cos(ωt), and the induced emf is N × B × A × ω × sin(ωt). Its peak value is N × B × A × ω. With 100 turns, 0.01 square meters, 0.5 tesla, and a 50 hertz rotation (ω = 2π × 50 ≈ 314 radians per second), the peak is 100 × 0.5 × 0.01 × 314 ≈ 157 volts. The voltage swings back and forth sinusoidally, which is alternating current.

Real power-plant generators use the opposite arrangement. A stationary set of wire coils, the stator, surrounds a rotating electromagnet, the rotor, and turning the rotor induces current in the coils. The EIA states that electromagnetic generators driven by mechanical prime movers account for nearly all U.S. electricity generation. The difference between plant types is what spins the rotor.

Plant type	What turns the generator
Coal, natural gas, nuclear, geothermal, biomass	Steam from heated water drives a steam turbine
Gas turbine plant	Hot combustion gas drives a combustion turbine
Hydroelectric	Falling water drives a water turbine
Wind	Wind pushes the blades of a turbine

There are important exceptions, and they are growing. Solar photovoltaic cells convert sunlight directly into electricity with no turbine and no induction, and fuel cells likewise make current without a spinning rotor. In 2024, solar supplied 6.9% of global electricity, according to Ember's Global Electricity Review 2025, so a statement like "every power plant uses induction" would be wrong. A better claim is that most of the world's electricity, from coal, gas, nuclear, hydro, and wind, is produced by generators that run on Faraday's principle. Hydro supplied 14.3%, nuclear 9.0%, and wind 8.1% in 2024 by the same review. For the broader story of how the grid grew from this, see the history of electricity and the electric power grid.

Motors: the same physics running in reverse

A generator turns motion into current. A motor turns current into motion. Strictly speaking, a motor does not use induction to make its torque. It uses the force that a magnetic field exerts on a current-carrying wire, the effect Ørsted and Ampère opened up and Faraday demonstrated in 1821 with his rotating wire. But the two phenomena are tied together. A spinning motor also acts as a generator, producing a back voltage that limits its current, and a motor designed for it can run backward to recover energy.

That second point is why electric vehicles can recover energy when braking. The U.S. Department of Energy notes that some vehicles use motor generators that perform both drive and regeneration functions. During braking, the motor resists the wheels' rotation and sends current back to the battery.

The scale is hard to overstate. According to a 2025 policy brief from the International Energy Agency's 4E program, electric motor systems accounted for 53% of global electricity consumption in 2023. Pumps, fans, compressors, conveyors, elevators, and factory machines are mostly motors in some form. Estimates vary with the year and with how motor systems are counted.

Transformers: why the grid works at all

A transformer is two coils on a shared iron core, which is Faraday's iron ring put to practical use. Alternating current in the primary coil produces a constantly changing flux in the core. That flux passes through the secondary coil and induces a voltage. For an ideal transformer, the ratio of the voltages equals the ratio of the turns: V primary ÷ V secondary = N primary ÷ N secondary.

Why does the grid need this? Because of the way power is lost in a wire. Power delivered is current times voltage, and the heat wasted in a wire is current squared times resistance. Send the same power at ten times the voltage, and the current drops by a factor of ten, so the heat loss drops by a factor of 100. A step-up transformer at the power plant raises the voltage for long-distance transmission, and step-down transformers near homes bring it back to a safe level.

Faraday's own ring was, as one history puts it, the first toroidal closed-core transformer, though he only applied pulses of current. The Irish priest Nicholas Callan built an induction coil in 1836 and noticed that more turns on the secondary gave a larger induced voltage. Practical power transformers appeared in the 1880s. Károly Zipernowsky, Ottó Bláthy, and Miksa Déri's team designed high-efficiency constant-potential transformers built at the Ganz factory in 1885, and Lucien Gaulard and John Dixon Gibbs sold their design to Westinghouse in the United States in 1886 (Wikipedia, Transformer). Typical distribution transformers today are between about 98 and 99 percent efficient.

This is the real story behind the nineteenth-century contest between alternating and direct current. A transformer needs a changing flux, so it works only with alternating current. Direct current could not be stepped up and down so simply, which gave AC the advantage for long-distance power. The people and politics are covered in Tesla's life and legacy, and the physics in how electricity works.

From lines of force to fields to light

Faraday's deepest contribution may be an idea rather than a machine. Before him, physicists generally pictured a force like a magnet's pull as acting across empty space, instantly, with nothing in between. Faraday thought of lines of force filling the space around a magnet or current, and of induction as a circuit being cut by or linked with those lines. Near the end of his career he proposed that electromagnetic forces extend into the space around a conductor, an idea his colleagues rejected during his lifetime.

Maxwell took the idea seriously. He translated Faraday's lines of force into mathematics and found that a changing electric field generates a magnetic field, just as Faraday had shown a changing magnetic field generates an electric one. The two feed each other, which means a disturbance can propagate through space as a wave. Maxwell sent his paper "A Dynamical Theory of the Electromagnetic Field" to the Royal Society in October 1864, where it was read on December 8, 1864, and published in 1865. His wave equation gave a speed in close agreement with measurements of the speed of light, and he concluded that "light is an electromagnetic disturbance propagated through the field according to electromagnetic laws."

The speed of light in vacuum is now defined as exactly 299,792,458 meters per second (CODATA 2022, NIST). That the same constant emerges from the electrical and magnetic properties of empty space is one of the great unifications in physics. The mathematics is explored in Maxwell's electromagnetic theory and in the biography James Clerk Maxwell, and the physics of magnets from atoms to planets is in how magnetism works.

Induction in everyday devices

Beyond generators, motors, and transformers, a long list of technologies rely on induction. Wikipedia's article on electromagnetic induction lists current clamps, induction cooking, induction motors, induction welding, inductive charging, near-field communication, magnetic flow meters, graphics tablets, and transcranial magnetic stimulation among them.

Induction cooktops run alternating current through a coil beneath the surface. The changing magnetic field induces eddy currents in the base of a compatible pot, and the pot's electrical resistance turns those currents into heat. The heat is created in the cookware itself, rather than transferred from a hot surface.

Wireless charging uses two coils. A coil in the charging pad carries alternating current and creates a changing field, and a coil in the phone picks up the induced voltage and feeds a charging circuit.

Metal detectors produce a changing magnetic field. A metal object in range has eddy currents induced in it, and those currents produce a field of their own that the detector senses.

MRI scanners are a looser relative. The imaging relies on nuclear magnetic resonance, not on induction as such, but the radio-frequency receiver coils pick up the faint signal from the body by induced voltage, as covered in how MRI machines work.

What people get wrong about induction

"A strong magnet near a wire makes a current." Only a changing flux does. A strong magnet sitting still next to a coil induces nothing, which is the observation that cost Faraday a decade.

"Induction creates energy." It converts it. Lenz's law means the induced current always resists the change that caused it, so you pay for every joule of electricity with mechanical work or another energy input.

"Faraday wrote Faraday's law." The experiments are Faraday's. The mathematical form of the law took shape in the 1840s with Neumann and others and was folded into Maxwell's theory later.

"Every power plant uses induction." Most do, but solar photovoltaic cells and fuel cells do not. Solar supplied 6.9% of global electricity in 2024.

"Faraday was an uneducated amateur." He had little formal schooling and little formal mathematics, but he worked as the director of the Royal Institution's laboratory from 1825 and held a professorship. His self-education was real, and so was his position.

Frequently Asked Questions

Q: What did Faraday actually discover in 1831? A: He discovered that a changing magnetic field induces a current in a nearby circuit. On August 29, 1831, switching current on and off in one coil produced brief currents in a second coil on the same iron ring, and soon afterward he got currents by moving a magnet in and out of a coil.

Q: Why does only a changing magnetic field create a current? A: Induced voltage depends on the rate of change of magnetic flux. A steady flux, however large, has zero rate of change and therefore induces zero voltage. A weak field that changes quickly can induce more than a strong field that changes slowly.

Q: What does the negative sign in Faraday's law mean? A: It expresses Lenz's law, formulated by Emil Lenz in 1834. The induced current flows in the direction that opposes the change in flux that caused it. This is required by conservation of energy, since otherwise induction could create energy from nothing.

Q: How do generators use electromagnetic induction? A: A generator rotates coils in a magnetic field, or a magnet inside coils. The rotation continuously changes the flux, which induces a voltage. In a power plant, steam, water, wind, or combustion gas turns the rotor, and the result is alternating current.

Q: Do all power plants rely on Faraday's discovery? A: Not all. Coal, gas, nuclear, hydro, and wind plants use generators based on induction. Solar photovoltaic panels and fuel cells produce electricity without it. In 2024, solar supplied 6.9% of global electricity, according to Ember.

Q: How are transformers related to induction? A: A transformer is two coils sharing an iron core. Alternating current in one coil makes a changing flux, which induces a voltage in the other. The voltage ratio equals the turns ratio, which lets power be sent at high voltage with low loss and then reduced for use.

Q: How much of the world's electricity do electric motors use? A: Electric motor systems accounted for 53% of global electricity consumption in 2023, according to a 2025 policy brief from the IEA's 4E program. Estimates vary with the year and the counting method.

The takeaway

Faraday's discovery rests on one counterintuitive rule: the world responds to change in a magnetic field, not to the field itself. Everything that follows is a way of arranging change on purpose, whether by spinning a coil, switching a current, or moving a magnet. Generators convert motion into electricity, transformers move that electricity across the country at low loss, and the closely related magnetic force on currents turns it back into motion in the motors that use over half of the world's electricity. Faraday did it with wire, an iron ring, and a needle, and the one thing he lacked was the mathematics, which Maxwell supplied.

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Read next:

• Electric Fields and Voltage Explained
• Galvani's Frog Leg Experiment: How a Dead Frog Started the Electrical Age
• The Radio: Marconi, Tesla, and the Invention That Wired the Air

Sources

• Faraday, M., Experimental Researches in Electricity, First Series, read to the Royal Society November 24, 1831 (Project Gutenberg edition).
• Royal Institution collection record for Faraday's induction ring.
• U.S. Energy Information Administration, How electricity is generated.
• Ember, Global Electricity Review 2025.
• IEA 4E Electric Motor Systems Annex, policy brief on why electric motor systems are important.
• NIST CODATA value of the speed of light in vacuum.
• Historical details on Faraday, Davy, Volta, Henry, Lenz, Neumann, Maxwell's 1864 paper, and transformer history: Wikipedia articles on Michael Faraday, Faraday's law of induction, Homopolar generator, Transformer, Humphry Davy, and A Dynamical Theory of the Electromagnetic Field.