CH. XXXIV.

CONSERVATION OF ENERGY.

339

height of 772 feet. This, you will notice, was exactly the converse of Joule's experiment, and proved that exactly as much motion is produced by means of confined heat as there is heat produced by means of checked motion.

Conservation of Energy.-And thus we arrive at one of the grandest discoveries of modern science, namely, that the whole amount of energy, or power of doing work, possessed by any set of bodies, remains unaltered whatever transformations it may undergo. It may exist in one of two forms either as potential or stored-up energy, which is unseen by us, or as visible energy, when it is actually performing work; but while it changes from one form to another its amount never alters. Thus in Joule's experiment the energy stored up in the weight which had been pulled up 772 feet was gradually transformed, as soon as the weight was released, into an amount of heat capable of raising the temperature of a pound of water 1° Fahr.; while Hirn showed, on the other hand, that exactly this amount of heat can be turned back into enough energy to raise a weight to the height of 772 feet at which it stood before.

The potential energy, or power of doing work, remained, therefore, exactly the same whether it was stored up in the weight or in the hot water, and reproduced exactly the same amount of visible energy or actual work. And even though we know that practically some energy disappears at every part of a machine when it is at work, yet this is not lost; for it turns into heat wherever it disappears as motion. If you grease the wheels of a machine, you will detect this heat beginning to do work again by turning the solid grease into a liquid.

By whatever means, therefore, heat is turned into motion, or motion into heat, the energy which causes them both

remains the same, and this is one out of many proofs that force cannot be destroyed, but is only lost in one form to reappear in another.

Other Experiments on Heat.-Although the experiments and calculations which have proved heat to be a kind of motion are some of the most interesting which have been made of late years, yet they are by no means the only ones. In 1811 Sir John Leslie carried on a most interesting series of observations on the reflection of heat; and the Italian physicist Melloni has traced the whole passage of heatrays through different solid bodies. All these discoveries are clearly and simply described in Professor Tyndall's work on 'Heat,' where you may also find the great additions that he has himself made to the work of these men.

We must content ourselves here with remembering that the physicists of the nineteenth century have shown that heat is a mode of motion,' and have traced it through all its many wanderings both in earth, air, and sky. They have even followed it from the sun down to our earth, through the plant-world into the beds of coal which are stored up in our rocks, and back again, when this coal is burnt, to the motion which carries our steam-engines and steam-ships across the world. All this lies before you to study in books of science, but now we must pass on to new discoveries in two remarkable sciences, namely, electricity and magnetism, which, we shall find, are closely bound up with heat and motion.

Chief Works consulted.-Rumford's Essays,' vol. ii.-Friction a Source of Heat,' 1798; Davy's 'Works,' vol. ii.-'Essay on Heat and Light;' Joule's 'Mechanical Equivalent of Heat' Phil. Trans.,' 1850; Mayer's 'Forces of Inorganic Nature'-'Phil. Mag.,' 1843; Tyndall's Heat a Mode of Motion;' Watts's 'Dict. of Chemistry,' art. 'Heat;' Clerk-Maxwell's 'Theory of Heat.'

CHI. XXXV.

OERSTED.

341

CHAPTER XXXV.

SCIENCE OF THE NINETEENTH CENTURY (CONTINUED). Oersted discovers the effect of Electricity upon a Magnet-Electromagnetism-Experiments by Ampère on Magnetic and Electric Currents-Ampère's Early Life-Direction of the North Pole of the Magnet depends on the course of the Electric Currents-Magnetic Currents set up between two Electric Wires - Electro-magnets made by means of an Electric Current-Arago magnetises a Steel Bar with an ordinary Electrical Machine-Faraday discovers the Rotatory Movement of Magnets and Electrified Wires-Produces an Electric Current by means of a Magnet-Seebeck discovers Thermo-electricity, or the production of Electricity by HeatSchwabe discovers Periodicity of the Spots on the Sun-Sabine suggests a connection between Sun-spots and Magnetic Currents-This proved in 1859 by observations of Carrington and HodgsonElectric Telegraph - Wheatstone — Cooke Steinheil-Morse-Bain.

Oersted discovers the Effect of Electricity upon a Magnet, 1820. We left the history of electricity at p. 264, at the point where Volta had shown in 1800 that two different metals joined by a wire and placed in acid and water will set up two currents of electricity flowing in opposite directions from one metal to the other along the wire, and back through the water. Every galvanic battery, that is, an apparatus for producing electricity by chemical action, is made on this principle. You will hear of Grove's battery, Bunsen's battery, Daniell's battery, and many others, all of which have been invented in the present century; but all

these are only different and more perfect methods of carrying out Volta's discovery. The next great step in the study of electricity was made by Oersted, Professor of Physics at Copenhagen, in 1820, twenty years after the invention of the voltaic pile.

Hans Christian Oersted was born in 1777, and died in 1851; he was a very eminent man, and wrote many works in Latin upon chemistry and magnetism, but the one discovery which has made him famous was that of electro-magnetism. We have seen (p. 53) that the invention of the mariner's compass in the fifteenth century arose from Flavio Gioja noting that a needle which has been rubbed along a piece of loadstone always points north and south. But why should the needle lie in this direction? What force makes it turn round when you leave it free after placing it another way? Ever since the fifteenth century people had asked this question, and when Volta and Franklin showed that electrical currents are constantly passing to and fro in our atmosphere, scientific men began to consider whether it might not be some force like electricity which acted upon the magnet ; especially as it had been observed that when a ship was struck by lightning, the needle of the mariner's compass was sometimes thrown quite out of its right position.

Still nothing was really known until the year 1819. In that year, when Professor Oersted was one day making some galvanic experiments at a lecture, it happened that a magnetic needle poised upon a point (as in Fig. 54) was standing near the wire along which an electric current was passing. All at once, when the current was very strong, the needle became excited and began to turn round upon the point. Oersted and his assistants were much surprised at this, and the consequence was that for several months Oersted made

CHI. XXXV.

ELECTRO MAGNETISM.

343

a series of experiments by which he proved that an electric current passing near a magnetic needle will always make it turn round so as to lie ACROSS the path of the current.

needle c will

For example, if the bar of copper wire a b, supported on the glass rods e, e, be so placed that the end points to the north and a to the south, then the magnetic lie exactly in a line with the bar, because a magnet always points north and south. But if the two ends of the copper rod a, b, are fixed to the wires of a voltaic battery d, Fig. 54, so that an electric current runs along the rod from a to b, then the north end of the needle will begin to move away from the north towards the west, that is towards the left side

FIG. 54.

Magnet turned by an Electric Current.

ab, Rod of copper wire. c, Magnetic needle. d, Voltaic pile (explained p. 263). ee, Glass supports to prevent the current running down to the ground.

of the current; and it will turn more and more as the current grows stronger, till it lies right across it, pointing direct east. and west.

This was a very grand fact, and it has become the beginning of a new science called electro-magnetism, for it shows that electricity and magnetism act upon each other in some peculiar way. Oersted did not publish an account of his experiments until 1820, and then the whole of Europe rang with the news of the discovery.

Ampère, 1775-1864.- One of the first men who heard