Physics 1831

Experimental Researches in Electricity (First Series)

Michael Faraday

Move a magnet past a coil and electricity flows — change itself is the source of the current.

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In depth · the introduction

For ten years Faraday chased one question — if electricity can make magnetism, can magnetism make electricity? — and in 1831 he found that the trick is motion.

The big idea

By 1820 it was known that an electric current makes a magnetic field: a current in a wire swings a nearby compass. Faraday was sure the reverse must also be true — that a magnet should be able to make a current — and he spent years trying. The answer, when it came, had a twist: a magnet sitting next to a coil does nothing at all. It is only while the magnet is moving, or while the magnetism is changing, that a current appears.

He proved it two ways. First with an iron ring carrying two separate coils: switching the current in one coil on or off made the galvanometer on the other coil jump — but only at the moment of switching. Then with no battery at all: pushing a bar magnet into a coil sent the needle one way, pulling it out sent it the other. Change was the whole secret.

How it came about

Faraday had almost no formal schooling. The son of a blacksmith, apprenticed to a bookbinder, he read the science in the books he was sewing, talked his way into a job as assistant to the chemist Humphry Davy at the Royal Institution, and became the greatest experimentalist of the age. In 1821 he had built the first electric motor, making a wire turn endlessly around a magnet. The reverse — making electricity from magnetism — took him another decade.

The breakthrough came on 29 August 1831 with the iron ring, and through the autumn he ran experiment after experiment, thrusting magnets into coils and spinning a copper disc between magnet poles to draw off a steady current. He read the work to the Royal Society that November. Across the Atlantic, the American Joseph Henry had seen much the same effect independently — but Faraday published first, and worked out what it meant.

Why it mattered

Faraday had found how to turn motion into electricity. Every power station today is a machine for doing exactly that — spinning magnets past coils, whether the spinning is driven by falling water, wind, steam, or a nuclear core. His iron ring became the transformer that carries power across whole countries. Without this single discovery there would be no electrical grid, and the wired world simply could not exist.

A way to picture it

Think of pushing a swing. Holding the swing still at any height does nothing; it is the movement that does the work, and the faster you push, the bigger the effect. A magnet near a coil is the same: park it there, even right inside, and not a flicker of current flows. Sweep it past, and current surges — and the quicker the sweep, the stronger the current. It is the change that pays, never the position.

Where it sits

Faraday's discovery is the hinge of the electromagnetic story. Hans Christian Ørsted (1820) had shown that current makes magnetism, and André-Marie Ampère had made that quantitative; Faraday found the missing reverse, that changing magnetism makes current. He pictured the space around magnets as filled with “lines of force” — a physical field — rather than action at a distance.

That picture was Faraday's deepest legacy. A generation later James Clerk Maxwell took these lines of force and wrote them as equations (his 1865 paper is also in this Library), folding Faraday's induction into the four laws that revealed light itself to be an electromagnetic wave. The self-taught bookbinder supplied the physical vision; Maxwell supplied the mathematics.

The original document

Original source text

On the induction of electric currents

Michael Faraday · Experimental Researches in Electricity, First Series · Phil. Trans. R. Soc. 122 (1832): 125–162 · read 24 November 1831

§1 · The plan

The power which electricity of tension possesses of causing an opposite electrical state in its vicinity has been expressed by the general term Induction; which, as it has been received into scientific language, may also, with propriety, be used in the same general sense to express the power which electrical currents may possess of inducing any particular state upon matter in their immediate neighbourhood, otherwise indifferent.

§6 · The first experiment

Two hundred and three feet of copper wire in one length were coiled round a large block of wood; other two hundred and three feet of similar wire were interposed as a spiral between the turns of the first coil, and metallic contact everywhere prevented by twine.

When the contact was made, there was a sudden and very slight effect at the galvanometer, and there was also a similar slight effect when the contact with the battery was broken. But whilst the voltaic current was continuing to pass through the one helix, no galvanometrical appearances nor any effect like induction upon the other helix could be perceived, although the active power of the battery was proved to be great.

[ … ]

Evolution of electricity from magnetism

First Series · the iron ring (§§27–31) and the moving magnet (§39)

Upon using the power of one hundred pairs of plates (10.) with this ring, the impulse at the galvanometer, when contact was completed or broken, was so great as to make the needle spin round rapidly four or five times, before the air and terrestrial magnetism could reduce its motion to mere oscillation.