The experiments already described in illustration of some of the phenomena of electro-magnetism are of such a simple nature that they may be comprehended without difficulty; but it is not such an easy task to appreciate the curious fact of an invisible power producing motion. It has already been explained that a copper or other metallic wire conveying a current of electricity becomes for the time endowed with a magnetic power, and if held above, or below, or close to, a suspended magnetized steel needle, affects it in a very marked degree, causing it to move to the right or left, according to the direction of the electric current; and in order to form some notion of the condition of a metallic wire whilst the electricity is passing through it, the annexed diagrams may be referred to. (Figs. 201, 202.)

Dr. Roget says: "The magnetic force which emanates from the electrical conducting wire is entirely different in its mode of operation from all other forces in nature with which we are acquainted. It does not act in a direction parallel to that of the current which is passing along the wire, nor in any plane passing through that direction. It is evidently exerted in a plane perpendicular to the wire, but still it has no tendency to move the poles of the magnet in a right or radial line, either directly towards, or directly from, the wire, as in every other case of attractive or repulsive agency. The peculiarity of its action is that it produces motion in a circular direction all round the wire—that is, in a direction at right angles to the radius, or in the direction of the tangent to a circle described round the wire in a plane perpendicular to it; hence the electro-magnetic force exerts a tangential action, or that which Dr. Wollaston called a vertiginous or whirling motion.

Dr. Faraday concluded that there is no real attraction or repulsion between the wire and either pole of a magnet, the action which imitates these effects being of a compound nature; and he also inferred that the wire ought to revolve round a magnetic pole of a bar magnet, and a magnetic pole round a wire, if proper means could be devised for giving effect to these tendencies, and for isolating the operations of a single pole. For the first idea of electro-magnetic rotation the world is indebted to Dr. Wollaston; but Dr. Faraday, with his usual ingenuity, was the first who carried out the theory practically. The rotation of a wire (conveying a current of voltaic electricity) round one of the poles of a magnet is well displayed with the simple contrivance devised by him. (Fig. 203.)

By a careful observation of the complex action of an electrified wire upon a magnetic needle, Dr. Faraday was enabled to analyse the phenomena with his usual penetration and exhaustive ability, and he found, as Daniell relates,—

"That if the electrified wire is placed in a perpendicular position, and made to approach towards one pole of the needle, the pole will not be simply attracted or repelled, but will make an effort to pass off on one side in a direction dependent upon the attractive or repulsive power of the pole; but if the wire be continually made to approach the centre of motion by either the one or the other side of the needle, the tendency to move in the former direction will first diminish, then become null, and ultimately the motion will be reversed, and the needle will principally endeavour to pass in the opposite direction. The opposite extremity of the needle will present similar phenomena in the opposite direction; hence Dr. Faraday drew the conclusion that the direction of the forces was tangential to the circumference of the wire, that the pole of the needle is drawn by one force, not in the direction of a radius to its centre, but in that of a line touching its circumference, and that it is repelled by the other force in the opposite direction. In this manner the northern force acted all round the wire in one direction, and the southern in the opposite one. Each pole of the needle, in short, appeared to have a tendency to revolve round the wire in a direction opposite to the other, and, consequently, the wire round the poles. Each pole has the power of acting upon the wire by itself, and not as connected with the opposite pole, and the apparent attractions and repulsions are merely exhibitions of the revolving motions in different parts of their circles."

The same fact illustrated at Fig. 203, is also demonstrated in a still more striking manner by means of wire bent into a rectangular form, and so arranged that whilst the current of electricity passes, it is free to move in a circle; and when the poles of a magnet are brought towards the electrified wire, it may be attracted or repelled at pleasure, and in fact becomes a magnetic indicator, and places itself (if carefully suspended) at right angles to the magnetic meridian. (Fig. 204.)

These curious movements of a magnetized needle, and rotations of wires and magnets, brought about by the agency of an active current of electricity, have induced Sir David Brewster to advance his admirable theory, which supposes the affection of the mariner's compass needle, and all other suspended pieces of steel, to be due to the agency of electrical currents continually circulating around the globe; and Mr. Barlow contrived the following experiment in illustration of Brewster's theory. A wooden globe, sixteen inches in diameter, was made hollow, for the purpose of reducing its weight, and while still in the lathe, grooves one-eighth of an inch deep and broad were cut to represent an equator, and parallels of latitude at every four and a half degrees each way from the equator to the poles. A groove of double depth was also cut like a meridian from pole to pole, but only half round. The grooves were cut to receive the copper wire covered with silk, and the laying on was commenced by taking the middle of a length of ninety feet of wire one-sixteenth of an inch in diameter, which was applied to the equatorial groove so as to meet in the transverse meridian; it was then made to pass round this parallel, returned again along the meridian to the next parallel, and then passed round this again, and so on, till the wire was thus led in continuation from pole to pole. The length of wire still remaining at each pole was returned from each pole along the meridian groove to the equator, and at this point, each wire being fastened down with small staples, the wires from the remaining five feet were bound together near their common extremity, when they opened to form separate connexions for the poles of a voltaic battery. When the battery was connected, and magnetic needles placed in different positions, they behaved precisely as they would do on the surface of the earth, the induction set up by the electrified wire being a perfect imitation of that which exists on the globe.

The opposite effect to that already described—viz., the rotation of one pole of a magnet round the electrified wire, was also arranged by Faraday in the following manner. (Fig. 205.)

In the examination of the magnetic phenomena obtained from wires transmitting a current of electricity, it should be borne in mind that any conducting medium which forms part of a closed circuit—i.e., any conductor, such as charcoal, saline fluids, acidulated water, which form a link in the endless chain required for the path of the electricity,—will cause a magnetic needle placed near it to deviate from its natural position.

These positions of the electrified wire and the magnetic needle are of course almost unlimited, and in order to assist the memory with respect to the fixed laws that govern these relative movements, Monsieur AmpÈre has suggested a most useful mechanical aid, and he says:—"Let the observer regard himself as the conductor, and suppose a positive electric current to pass from his head towards his feet, in a direction parallel to a magnet; then its north pole in front of him will move to his right side, and its south pole to his left.

"The plane in which the magnet moves is always parallel to the plane in which the observer supposes himself to be placed. If the plane of his chest is horizontal, the plane of the magnet's motion will be horizontal, but if he lie on either side of the horizontally-suspended magnet, his face being towards it, the plane of his chest will be vertical, and the magnet will tend to move in a vertical plane."

This very lucid comparison will be seen to apply perfectly to the direction of the rotations in Figs. 203 and 205.

The whole of this apparatus is made in the most elegant and finished manner by Messrs. Elliott, of 30, Strand; and by a modification of the latter arrangement (Fig. 206), the opposite rotations of the opposite poles of the magnets round the electrified wire, are shown in the most instructive manner. The apparatus (Fig. 206) was devised by the late Mr. Francis Watkins, and consists of two flat bar magnets doubly bent in the middle, and having agate cups fixed at the under part of the bend (by which they are supported) upon upright pointed wires, the latter being fixed upright on the wooden base of the apparatus, and the magnets turn round them as upon an axis.

Two circular boxwood cisterns, to contain quicksilver, are supported upon the stage or shelf above the base. A bent pointed wire is directed into the cup of each magnet, the ends of which dip into the mercury contained in the boxwood circular troughs on the stage. By using a battery to each magnet, and taking care that the currents of electricity flow precisely alike, they will then rotate in opposite directions.

Directly after the ingenious experiments of Faraday became known, a great number of electro-magnetic engine models were constructed, and many thought that the time was fast approaching when steam would be superseded by electricity; and really, to see the pretty electro-magnetic models work with such amazing rapidity, it might be supposed that if they were constructed on a larger scale, a great amount of hard work could be obtained from them. This idea, however, has been proved to be a fallacy, for reasons that will be presently explained. The figure on p. 216 displays two of these engines, one of which represents the rotation of electro-magnets within four fixed steel magnets, and the other the rotation of steel magnets by the fixed electro-magnets. The latter (No. 2) moves with such great velocity, that unless the strength of the battery is carefully adjusted, the connexions are soon destroyed. (Fig. 207.)

Considering the prodigious power or pull of a soft-iron electro-magnet, and its capability of supporting considerable weight, the most reasonable expectations of success might be entertained with machines acting by the direct pull. It was, however, discovered that they soon became inefficient, from the circumstance that the repeated blows received by the iron so altered its character, that it eventually assumed the quality of steel, and had a tendency to retain a certain amount of permanent magnetism, and thus to interfere with the principle of making and unmaking a magnet. It was this fact that induced Professor Jacobi, of St. Petersburg, after a large expenditure of money, to abandon arrangements of this kind, and to employ such as would at once produce a rotatory motion. The engine thus arranged was tried upon a tolerably large scale on the Neva, and by it a boat containing ten or twelve people was propelled at the rate of three miles an hour.

Various engines have been constructed by Watkins, Botta, Jacobi, Armstrong, Page, Hjorth; the engine made by the latter (Hjorth) excited much attention in 1851-52, and consisted of an electro-magnetic piston drawn within or repelled from an electro-magnetic cylinder; and by this motion it was thought that a much greater length of stroke could be secured than by the revolving wheels or discs, but the loss of power (not only in this engine, but in others) through space is very great, and the lifting power of any magnet is greatly reduced and altered at the smallest possible distance from its poles. This loss of power is therefore a great obstacle in the way of the useful application of electro-magnetic force, and can be appreciated even with the little models, all of which may be stopped with the slightest friction, although they may be moving at the time with great velocity.

In the second place, supposing the reduced force exerted by the two magnets, a few lines apart, was considered available for driving machinery, the moment the magnets begin to move in front of one another there is again a great loss of power, and as the speed increases, there is curiously a corresponding diminution of available mechanical power, a falling-off in the duty of the engine as the rotations become more rapid. In the third place, the cost of the voltaic battery, as compared with the consumption of coal in the steam-engine, is very startling, and extremely unfavourable to electro-magnetic engines.

Mr. J. P. Joule found that the economical duty of an electro-magnetic engine at a given velocity and for a given resistance of the battery is proportioned to the mean intensity of the several pairs of the battery. With his apparatus, every pound of zinc consumed in a Grove's battery produced a mechanical force (friction included) equal to raise a weight of 331,400 pounds to the height of one foot, when the revolving magnets were moving at the velocity of eight feet per second. Now, the duty of the best Cornish steam-engine is about one million five hundred thousand pounds raised to the height of one foot by the combustion of each pound of coal, or nearly five times the extreme duty that could be obtained from an electro-magnetic engine by the consumption of one pound of zinc. This comparison is therefore so very unfavourable, that the idea of a successful application of electricity as an economic source of power, is almost, if not entirely abandoned.

By instituting a comparison between the different means of producing power, it has been shown that for every shilling expended there might be raised by

A powerful magnet has been compared to a steam-engine with an enormous piston but with an exceedingly short stroke. Although motive power cannot be produced from electricity and applied successfully to commercial purposes, like the steam-engine, yet the achievements of the electric telegraph as an application of a small motive power must not be lost sight of, whilst the fall of the ball at Deal and other places, by which the chronometers of the mercantile navy are regulated, as also the means of regulating the time at the General Post Office and various railway stations, are all useful applications of the power which fails to compete in other ways with steam.