The Boy's Playbook of Science Part 26

_Third Experiment._

If another bar of steel is magnetized, and the north pole duly marked, and then brought towards the same pole of the suspended magnet, instant repulsion takes place; the magnet, of course, grasped in the hand is not free to move, but the small magnet immediately shows the same fact noticed with electricity, viz., "_that similar magnetisms repel_." Two north poles repel each other, but when the bar of steel is reversed, the opposite effect occurs, and the suspended magnet is attracted, showing that _dissimilar magnetisms attract_, and a north will attract a south pole. (Fig. 197.)

[Ill.u.s.tration: Fig. 197. A magnetic needle, the north pole N being attracted to the south pole of the bar magnet S, and repelled from the north end.]

_Fourth Experiment._

[Ill.u.s.tration: Fig. 198. The horse-shoe magnet, and another one unmagnetized, placed end to end; the one shaded and lettered N and S is the magnet. A A. The piece of soft iron moved in the direction of the arrow.]

By contact, the magnetic power is transferred from the magnet to a piece of unmagnetized steel, and it is stated that the highest magnetizing effect is that produced by the simple method of Jacobi. A horse-shoe magnet has its poles brought in contact with the intended poles of another bar of steel, likewise bent in the form of a horse-shoe, and by [Page 209] drawing the feeder over the unmagnetized horse-shoe in the direction of the arrow in the cut, and when it reaches the curve, bringing it back again to the same place, say at least twelve times, and after turning the whole over without separating the poles, and repeating the same operation on the other side likewise twelve times, the steel is then powerfully magnetized; and it is said that a horse-shoe of one pound weight may be thus charged so as to sustain twenty-six and a half pounds, and that by the old method of magnetizing it would only have sustained about twenty-two pounds. (Fig. 198.)

_Fifth Experiment._

If the horse-shoe magnet is placed on a sheet of paper, and some iron filings are dusted between the poles, a very beautiful series of curves are formed, called the magnetic curves, which indicate the constant pa.s.sage of the magnetic power from pole to pole.

_Sixth Experiment._

The magnetic force exerted by a horse-shoe-shaped piece of soft iron, surrounded with many strands of covered copper wire in short lengths, is extremely powerful (Fig. 199), and enormous weights have been supported by an electro-magnet when connected with a voltaic battery. Supposing a man were dressed in complete armour, he might be held by an electro-magnet, without the power of disengaging himself, thus realizing the fairy story of the bold knight who was caught by a rock of loadstone, and, in full armour, detained by the unfriendly magician.

[Ill.u.s.tration: Fig. 199. A. Powerful electro-magnet supporting a great weight. B. The battery.]

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_Seventh Experiment._

When a piece of soft iron is held sufficiently near one of the poles of a powerful magnet, it becomes by _induction_ endowed with magnetic poles, and will support another bit of soft iron, such as a nail, brought in contact with it. When the magnet is removed, the inductive action ceases, and the soft iron loses its magnetic power. This experiment affords another example of the connexion between the phenomena of electricity and magnetism. It is in consequence of the inductive action of the magnetism of the earth that all ma.s.ses of iron, especially when they are perpendicular, are found to be endowed with magnetic polarity; hence the reaction of the iron in s.h.i.+ps upon the compa.s.ses, which have to be corrected and adjusted before a voyage, or else serious errors in steering the vessel would occur, and there is no doubt that many s.h.i.+pwrecks are due to this cause. No other metals beside iron, steel, nickel, cobalt, and possibly manganese, can receive or retain magnetism after contact with a magnet.

The remarkable effect of magnetism upon all matter, so ably investigated by Faraday and others, will be explained in another part of this book--viz., in the article on Dia-Magnetism.

[Ill.u.s.tration: Fig. 200. Magician and his loadstone-rock.--Vide _Fairy Tale_.]

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CHAPTER XVI.

ELECTRO-MAGNETIC MACHINES.

The experiments already described in ill.u.s.tration 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 pa.s.sing through it, the annexed diagrams may be referred to. (Figs. 201, 202.)

[Ill.u.s.tration: Fig. 201. Portion of a square copper conductor, in which A B represents the direction of the electricity, and the small arrows, C C C C, the magnetic current or whirl at right angles to the electrical current, and exercising a tangential action.]

[Ill.u.s.tration: Fig. 202. A round conducting wire, in which the electrical current is flowing in the direction of the large dart A B, and the small arrows indicate the direction of the magnetic force.]

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 pa.s.sing along the wire, nor in any plane pa.s.sing 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."

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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.)

[Ill.u.s.tration: Fig. 203. N. A small bar magnet cemented into a wine-gla.s.s, the north pole being at N. A is a moveable wire looped over the hook, which is the positive (+) pole of the battery; the free extremity rotates round the pole of the magnet when the current of electricity pa.s.ses. The dotted line represents the level of the mercury which the gla.s.s contains. The electricity pa.s.ses in at A, and out at the wire B, as shown by the arrows. C is connected with the negative, and D with the positive, pole of the battery.]

By a careful observation of the complex action of an electrified wire upon a magnetic needle, Dr. Faraday was enabled to a.n.a.lyse 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 pa.s.s 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 princ.i.p.ally endeavour to pa.s.s 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 circ.u.mference 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 circ.u.mference, 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."

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The same fact ill.u.s.trated 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 pa.s.ses, 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.)

[Ill.u.s.tration: Fig. 204. A A A A. The rectangular wire covered with silk and varnished, one end of which being pointed, rests on the little cup B, connected with a covered wire pa.s.sing down the centre of the bra.s.s support to the binding screw C let into ivory. D. The other extremity of the rectangular wire; this being covered and varnished, is not in metallic contact with the end B, but is likewise pointed, and dips into the mercury contained in the large cup E E. The upper and lower cups do not touch, and are separated by ivory, marked by the shaded portion, and the cup E E is in metallic communication with the bra.s.s pillar, and is connected with the negative pole of the battery at F, whilst C is connected with the positive pole of the battery, and the electricity circulates round the wire in the direction of the arrows. When a bar magnet, N, is brought towards the wire, the latter is immediately set in motion, and by alternately presenting the opposite poles of the magnet, the rectangular wire rotates freely round the cup B.]

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 compa.s.s 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 ill.u.s.tration 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 lat.i.tude 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 pa.s.s round this parallel, returned again along the meridian to the next parallel, and then pa.s.sed round this again, and so on, till the wire was thus led in continuation from pole [Page 214] 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.)

[Ill.u.s.tration: Fig. 205. N S. A little magnet floating in mercury contained in the gla.s.s A A; the north pole is allowed to float above the surface of the quicksilver, and the south pole is attached to the wire pa.s.sing through the bottom of the gla.s.s vessel. The electricity pa.s.ses in at B, and taking the course indicated by the arrows travels through the gla.s.s of quicksilver to the other pole of the battery at C. Directly contact is made with the battery, the little magnet rotates round the electrified wire, W. The dotted line shows the level of the mercury in gla.s.s.]

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 a.s.sist the memory with respect to the fixed laws that govern these relative movements, Monsieur Ampere 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 pa.s.s 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 [Page 215] 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.

[Ill.u.s.tration: Fig. 206. A. Wire conveying the current of electricity. B B. The magnets balanced on points rotating round the wires.]

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.)

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