Professor Arago, whom we mentioned before (p. 311) as making experiments on light, succeeded in magnetising a steel bar with currents from an ordinary electrical machine, that is, a glass cylinder rubbed against silk, instead of using a battery.

Michael Faraday, 1791-1867.-We must now travel back to England, where one of our greatest philosophers was watching these new discoveries with intense interest. Michael Faraday, the son of a poor journeyman blacksmith, was born at Newington Butts in 1791. When he was thirteen years old he went as errand boy to a bookseller named Mr. Rieban, in Blandford Street, Manchester Square, and it was there that the books fell into his hands which first awoke his love of science. Mrs. Marcet's Conversations on Chemistry,' Lyons's 'Experiments on Electricity,' and other books of a like kind made the lad long for more knowledge about these wonderful sciences. He constructed an electrical machine, and spent his evenings in making experiments, and he persuaded his brother Robert to pay a few shillings for him to attend some lectures given by a Mr. Tatum on Natural Philosophy.

But one of the first great pleasures of his life was when a customer at the bookshop, a Mr. Dance, took him to four lectures at the Royal Institution, given by Sir Humphry Davy. These lectures filled him with an intense longing to learn more, and he took the bold step of writing a letter to Davy, enclosing the notes which he had made of the lectures, and asking for some employment connected with science. It will always be remembered to Davy's honour that he did not throw this letter aside, but wrote a kind reply, telling the young man to come and see him, and in the end made him his assistant at the Royal Institution

CH. XXXV.

FARADAY.

349

in Albemarle Street, where Faraday afterwards became Professor of Chemistry.

It is impossible in a short sketch to give you any idea of the simple and noble nature of the man who from that time for more than fifty years laboured at science in the Royal Institution. It is not yet eight years since he died, and you may talk with many who have known and loved him, and if you wish to learn the story of his life you must read it in the book called 'Michael Faraday,' written by Dr. Gladstone. Even of his experiments we can only mention a few, for these subjects are becoming almost too deep for us; but those which we must now consider were some which have helped to make his name famous.

Faraday discovers the Mutual Rotation of Magnets and Electrified Wires, 1821.-It was in 1821 that Faraday began to repeat for himself Ampère's experiments on electricity and magnetism, and he soon saw that if an electric current going round a wire gave rise to magnetic currents at right angles to it, he ought to be able to make an electric wire revolve round a magnet, and a magnet round an electric wire. Accordingly, he took two cups of mercury, A B, Fig. 58, p. 350, and drilling a hole in the bottom of each, he passed the wires e, e', of a battery up into them; then he took two magnets d, d; dhe fastened by a thin thread to the battery wire in the cup A, so that it floated upright in the mercury, and the top of it could move round easily; the other magnet, ď, he fixed firmly upright in the cup B. He then hung the copper rod c above the cups, so that the end f, which was fixed, dipped into the cup A, and the other end, which was made of a loose moveable wire, ƒ', dipped into the cup B. Thus in A the magnet was free to move and the wire was fixed, while in в the wire was free to move

and the magnet was fixed. He now sent a current through the wires, e, and immediately in the cup A the magnet d began to move round the fixed wire f, while in в the wire f' moved round the fixed magnet, d'. In this way he proved that magnetic and electric currents move round and round in circles at right angles to each other. He made the magnet go

(Brande).

A B, Section of cups of mercury. c, Copper rod. The current coming in at e passes up through the mercury in A, and along the rod c down into the mercury in B, and back bye to the battery. On its way it causes the floating magnet, d, to revolve round the rod, f, and the loose wire, f, to revolve round the fixed magnet, d.

a great way round the circle, but not spin quite round as the wire had done. Ampère, however, who repeated the experiment, succeeded in making the magnet spin round and round like the hands of a clock.

Electric Current produced by means of a Magnet.Faraday's mind was now full of the wonderful effect which electricity and magnetism produce on each other, and he began to consider whether it might not be possible to reverse Ampère's second experiment (p. 347), and instead of making a magnet by means of an electric current, whether he might not set up an electric current by means of a magnet.

CH. XXXV.

THE INDUCTION COIL.

351

To try this he wound from 200 to 300 yards of wire round a hollow cylinder a, Fig. 59, and carried the two ends of the wire to a little instrument b, called a galvanometer, which was invented by Ampère, and the needle of which moves directly the slightest current passes through it. He then took a powerful bar magnet, c, and held it within the cylinder. The moment he put it in, the needle of the galvanometer showed that an electric current had passed through

a,

(Ganot).

Coil of wire round a wooden cylinder connected at the two ends with b, a galvanometer, the needle of which shows directly a current passes through the wire; c, a powerful magnet.

the wire in one direction, and the moment he drew it out another rush of electricity occurred in the other direction, showing that the magnet had set up an electric current in a coil of wire. While the magnet remained in the cylinder there was no current; it was only at the moment of going in and coming out that it produced the effect. By a more complicated apparatus Faraday succeeded in making these currents strong enough to produce electric sparks; and it is

on this principle that the induction-coil is made which is now used to increase the power of the electricity coming from an electric battery.

Professor Seebeck discovers Thermo-electricity, or the Production of Electricity by Heat.-The fact was now clearly established that electric and magnetic currents move at right angles to each other, and this gives to a certain extent the answer to our question. Why does a magnet turn to the north? Ampère suggested quite early in the discussion, that if an electric current will turn metals into magnets, the electric currents which we know are flowing from east to west round our globe, may turn the earth (which is full of metals) into a great magnet. But it is also true that exactly the opposite effect is possible, and that the magnetic currents may be started by some other cause and may set up the electric currents, so that we do not really know which gives rise to the other.

He wished to

An interesting discovery was, however, made in 1822 by Professor Seebeck, showing a possible cause of the electric currents flowing from east to west. try whether he could not give rise to a current of electricity in two metals by merely using heat instead of acid and water. For this purpose he took a half ring of copper and fastened to it a bar of a metal called antimony, so that the two metals had the form of a stirrup, and inside this stirrup he hung a magnetic needle, which would show if any current passed along the metals. Then he heated one of the corners where the metals joined, and immediately the magnet began to turn, showing that an electric current was passing through the copper, and back through the antimony. He tried this with many other metals, and in every case when one of the parts where they joined was made hotter