Part 14 (2/2)

Maxwell showed, too, that Faraday's electro-tonic state, on the variation of which induced currents depend, corresponds completely with the number of lines of magnetic force pa.s.sing through the circuit.

He also showed that, when a conductor conveying a current is free to move in a magnetic field, or magnets are free to move in the neighbourhood of such a conductor, _the system will a.s.sume that condition in which the greatest possible number of lines of magnetic force pa.s.s through the circuit in the positive direction_.

But Maxwell was not content with showing that Faraday's conceptions were consistent, and had their mathematical equivalents,--he proceeded to point out how a medium could be imagined so const.i.tuted as to be able to perform all the various duties which were thus thrown upon it.

a.s.suming a medium to be made up of spherical, or nearly spherical, cells, and that, when magnetic force is transmitted, these cells are made to rotate about diameters coinciding in direction with the lines of force, the tension along those lines, and the pressure at right angles to them, are accounted for by the tendency of a rotating elastic sphere to contract along its polar axis and expand equatorially so as to form an oblate spheroid. By supposing minute spherical particles to exist between the rotating cells, the motion of one may be transmitted in the same direction to the next, and these particles may be supposed to const.i.tute electricity, and roll as perfectly rough bodies on the cells in contact with them. Maxwell further imagined the rotating cells, and therefore, _a fortiori_, the electrical particles, to be extremely small compared with molecules of matter; and that, in conductors, the electrical particles could pa.s.s from molecule to molecule, though opposed by friction, but that in insulators no such transference was possible. The machinery was then complete. If the electric particles were made to flow in a conductor in one direction, pa.s.sing between the cells, or _molecular vortices_, they compelled them to rotate, and the rotation was communicated from cell to cell in expanding circles by the electric particles, acting as idle wheels, between them. Thus rings of magnetic force were made to surround the current, and to continue as long as the current lasted.

If an attempt were made to displace the electric particles in a dielectric, they would move only within the substance of each molecule, and not from molecule to molecule, and thus the cells would be deformed, though no continuous motion would result. The deformation of the cells would involve elastic stress in the medium. Again, if a stream of electric particles were started into motion, and if there were another stream of particles in the neighbourhood free to flow, though resisted by friction, these particles, instead of at once transmitting the rotary motion of the cells on one side of them to the cells on the other side, would at first, on account of the inertia of the cells, begin to move themselves with a motion of translation opposite to that of the primary current, and the motion would only gradually be destroyed by the frictional resistance and the molecular vortices on the other side made to revolve with their full velocity. A similar effect, but in the opposite direction, would take place if the primary current ceased, the vortices not stopping all at once if there were any possibility of their continuing in motion. The imaginary medium thus serves for the production of induced currents.

The mechanical forces between currents and magnets and between currents and currents, as well as between magnets and currents, were accounted for by the tension and pressure produced by the molecular vortices. When currents are flowing in the same direction in neighbouring conductors, the vortices in the s.p.a.ce between them are urged in opposite directions by the two currents, and remain almost at rest; the lateral pressure exerted by those on the outside of the conductors is thus unbalanced, and the conductors are pushed together as though they attracted each other. When the currents flow in opposite directions in parallel conductors, they conspire to give a greater velocity to the vortices in the s.p.a.ce between them, than to those outside them, and are thus pushed apart by the pressure due to the rotation of the vortices, as though they repelled each other. In a similar way, the actions of magnets on conductors conveying currents may be explained. The motion of a conductor across a series of lines of magnetic force may squeeze together and lengthen the threads of vortices in front, and thus increase their speed of rotation, while the vortices behind will move more slowly because allowed to contract axially and expand transversely. The velocity of the vortices thus being greater on one side of the wire than the other, a current must be induced in the wire. Thus the current induced by the motion of a conductor in a magnetic field may be accounted for.

This conception of a medium was given by Maxwell, not as a theory, but to show that it was possible to devise a _mechanism_ capable, in imagination at least, of producing all the phenomena of electricity and magnetism. ”According to our theory, the particles which form the part.i.tions between the cells const.i.tute the matter of electricity. The motion of these particles const.i.tutes an electric current; the tangential force with which the particles are pressed by the matter of the cells is electro-motive force; and the pressure of the particles on each other corresponds to the tension or potential of the electricity.”

When a current is maintained in a wire, the molecular vortices in the surrounding s.p.a.ce are kept in uniform motion; but if an attempt be made to stop the current, since this would necessitate the stoppage of the vortices, it is clear that it cannot take place suddenly, but the energy of the vortices must be in some way used up. For the same reason it is impossible for a current to be suddenly started by a finite force. Thus the phenomena of self-induction are accounted for by the supposed medium.

The magnetic permeability of a medium Maxwell identified with the density of the substance composing the rotating cells, and the specific inductive capacity he showed to be inversely proportional to its elasticity. He then proved that the ratio of the electro-magnetic unit to the electro-static unit must be equal to the velocity of transmission of a transverse vibration in the medium, and consequently proportional to the square root of the elasticity, and inversely proportional to the square root of the density. If the medium is the same as that engaged in the propagation of light, then this ratio ought to be equal to the velocity of light, and, moreover, in non-magnetic media, the refractive index should be proportional to the square root of the specific inductive capacity. The different measurements which had been made of the ratio of the electrical units gave a mean very nearly coinciding with the best determinations of the velocity of light, and thus the truth underlying Maxwell's speculation was strikingly confirmed, for the velocity of light was determined by purely electrical measurements. In the case also of bodies whose chemical structure was not very complicated, the refractive index was found to agree fairly well with the square root of the specific inductive capacity; but the phenomenon of ”residual charge” rendered the accurate measurement of the latter quant.i.ty a matter of great difficulty. It therefore appeared highly probable that light is an electro-magnetic disturbance due to a motion of the electric particles in an insulating medium producing a strain in the medium, which becomes propagated from particle to particle to an indefinite distance. In the case of a conductor, the electric particles so displaced would pa.s.s from molecule to molecule against a frictional resistance, and thus dissipate the energy of the disturbance, so that true (_i.e._ metallic) conductors must be nearly impervious to light; and this also agrees with experience.

Maxwell thus furnished a complete theory of electrical and electro-magnetic action in which all the effects are due to actions propagated in a medium, and direct action at a distance is dispensed with, and exposed his theory successfully to most severe tests. In his great work on electricity and magnetism, he gives the mathematical theory of all the above actions, without, however, committing himself to any particular form of mechanism to represent the const.i.tution of the medium. ”This part of that book,” Professor Tait says, ”is one of the most splendid monuments ever raised by the genius of a single individual.... There seems to be no longer any possibility of doubt that Maxwell has taken the first grand step towards the discovery of the true nature of electrical phenomena. Had he done nothing but this, his fame would have been secured for all time. But, striking as it is, this forms only one small part of the contents of this marvellous work.”

CONCLUSION.

SOME OF THE RESULTS OF FARADAY'S DISCOVERIES, AND THE PRINCIPLE OF ENERGY.

In early days, _the spirit of the amber_, when aroused by rubbing, came forth and took to itself such light objects as it could easily lift. Later on, and the spirit gave place to the _electric effluvium_, which proceeded from the excited, or charged, body into the surrounding s.p.a.ce. Still later, and a fluid, or two fluids, acting directly upon itself, or upon matter, or on one another, through intervening s.p.a.ce without the aid of intermediate mechanism, took the place of the electric effluvium--a step which in itself was, perhaps, hardly an advance. Then came the time for accurate measurement. The simple _observation_ of phenomena and of the results of experiment must be the first step in science, and its importance cannot be over-estimated; but before any quant.i.ty can be said to be known, we must have learned how to _measure_ it and to reproduce it in definite amounts. The great law of electrical action, the same as that of gravitation--the law of the inverse square--soon followed, as well as the a.s.sociated fact that the electrification of a conductor resides wholly on its surface, and there only in a layer whose thickness is too small to be discovered. The fundamental laws of electricity having thus been established, there was no limit to the application of mathematical methods to the problems of the science, and, in the hands of the French mathematicians, the theory made rapid advances. George Green, of Sneinton, Nottingham, introduced the term ”potential” in an essay published by subscription, in Nottingham, in 1828, and to him we are indebted for some of our most powerful a.n.a.lytical methods of dealing with the subject; but his work remained unappreciated and almost unknown until many of his theorems had been rediscovered. But the idea of a body acting where it is not, and without any conceivable mechanism to connect it with that upon which it operates, is repulsive to the minds of most; and, however well such a theory may lend itself to mathematical treatment and its consequences be borne out by experiment, we still feel that we have not solved the problem until we have traced out the hidden mechanism. The pull of the bell-rope is followed by the tinkling of the distant bell, but the young philosopher is not satisfied with such knowledge, but must learn ”what is the particular go of that.” This universal desire found its exponent in Faraday, whose imagination beheld ”lines” or ”tubes of force” connecting every body with every other body on which it acted.

To his mind these lines or tubes had just as real an existence as the bell-wire, and were far better adapted to their special purposes.

Maxwell, as we have seen, not only showed that Faraday's system admitted of the same rigorous mathematical treatment as the older theory, and stood the test as well, but he gave reality to Faraday's views by picturing a mechanism capable of doing all that Faraday required of it, and of transmitting light as well. Thus the problem of electric, magnetic, and electro-magnetic actions was reduced to that of strains and stresses in a medium the const.i.tution of which was pictured to the imagination. Were this theory verified, we might say that we know at least as much about these actions as we know about the transmission of pressure or tension through a solid.

With regard to the _nature_ of electricity, it must be admitted that our knowledge is chiefly negative; but, before deploring this, it is worth while to inquire what we mean by saying that we know what a thing is. A definition describes a thing in terms of other things simpler, or more familiar to us, than itself. If, for instance, we say that heat is a form of energy, we know at once its relations.h.i.+p to matter and to motion, and are content; we have described the const.i.tution of heat in terms of simpler things, which are more familiar to us, and of which we _think_ we know the nature. But if we ask what _matter_ is, we are unable to define it in terms of anything simpler than itself, and can only trust to daily experience to teach us more and more of its properties; unless, indeed, we accept the theory of the vortex atoms of Thomson and Helmholtz. This theory, which has recently been considerably extended by Professor J. J.

Thomson, the present occupier of Clerk Maxwell's chair in the University of Cambridge, supposes the existence of a perfect fluid, filling all s.p.a.ce, in which minute whirlpools, or vortices, which in a perfect fluid can be created or destroyed only by superhuman agency, form material atoms. These are _atoms_, that is to say, they defy any attempts to sever them, not because they are infinitely hard, but because they have an infinite capacity for _wriggling_, and thus avoid direct contact with any other atoms that come in their way. Perhaps a theory of electricity consistent with this theory of matter may be developed in the future; but, setting aside these theories, we may possibly say that we know as much about electricity as we know about matter; for while we are conversant with many of the properties of each, we _know_ nothing of the ultimate nature of either.

But while the theory of electricity has scarcely advanced beyond the point at which it was left by Clerk Maxwell, the practical applications of the science have experienced great developments of late years. Less than a century ago the lightning-rod was the only practical outcome of electrical investigations which could be said to have any real value. [OE]rsted's discovery, in 1820, of the action of a current on a magnet, led, in the hands of Wheatstone, Cooke, and others, to the development of the electric telegraph. Sir William Thomson's employment of a beam of light reflected from a tiny mirror attached to the magnet of the galvanometer enabled signals to be read when only extremely feeble currents were available, and thus rendered submarine telegraphy possible through very great distances. The discovery by Arago and Davy, that a current of electricity flowing in a coil surrounding an iron bar would convert the bar into a magnet, at once rendered possible a variety of contrivances whereby a current of electricity could be employed to produce small reciprocating movements, or even continuous rotation, where not much power was required, at a distance from the battery. An ill.u.s.tration of the former is found in the common electric bell; it is only necessary that the vibrating armature should form part of the circuit of the electro-magnet, and be so arranged that, while it is held away from the magnet by a spring, it completes the battery circuit, but breaks the connection as soon as it moves towards the magnet under the magnetic attraction. To produce continuous rotation, a number of iron bars may be attached to a fly-wheel, and pa.s.s very close to the poles of the magnet without touching them; when a bar is near the magnet, and approaching it, contact should be made in the circuit, but should be broken, so that the magnet may lose its power, as soon as the bar has pa.s.sed the poles; or the continuous rotation may be produced from an oscillating armature by any of the mechanical contrivances usually adopted for the conversion of reciprocating into continuous circular motion. But all such motors are extremely wasteful in their employment of energy. Faraday's discovery of the rotation of a wire around a magnetic pole laid the foundation for a great variety of electro-motors, in some of which the efficiency has attained a very high standard. About ten years ago, Clerk Maxwell said that the greatest discovery of recent times was the ”reversibility” of the Gramme machine, that is, the possibility of causing the armature to rotate between the field-magnets by sending a current through the coils. The electro-motors of to-day differ but little from dynamos in the principles of their construction. The copper disc spinning between the poles of a magnet while an electric current was sent from the centre to the circ.u.mference, or _vice versa_, formed the simplest electro-motor. All the later motors are simply modifications of this, designed to increase the efficiency or power of the machine.

Similarly, the earliest machine for the production of an electric current at the expense of mechanical power only, but through the intervention of a permanent magnet, was the rotating disc of Faraday, described on page 262. This contrivance, however, caused a waste of nearly all the energy employed, for while there was an electro-motive force from the centre to the circ.u.mference, or in the reverse direction, in that part of the disc which was pa.s.sing between the poles of the magnet, the current so generated found its readiest return path through the other portions of the disc, and very little traversed the galvanometer or other external circuit. This source of waste could be, for the most part, got rid of by cutting the disc into a number of separate rays, or spokes, and filling up the s.p.a.ces between them with insulating material. The current then generated in the disc would be obliged to complete its circuit through the external conductor. If we can so arrange matters as to employ at once several turns of a continuous wire in place of one arm, or ray, of the copper disc, we may multiply in a corresponding manner the electro-motive force induced by a given speed of rotation. All magneto-electric generators are simply contrivances with this object. The iron cores frequently employed within the coils of the armature tend to concentrate the lines of force of the magnet, causing a greater number to pa.s.s through the coils in certain positions than would pa.s.s through them were no iron present. The electro-motive force of such a generator depends on the strength of the magnetic field, the length of wire employed in cutting the lines of force, and the speed with which the wire moves across these lines. The point to aim at in constructing an armature is to make the resistance as small as possible consistent with the electro-motive force required. As there is a limit to the strength of the magnetic field, it follows that for strong currents, where thick wire must be employed, the generator must be made of large dimensions, or the armature must be driven at very high speed to enable a shorter length of wire to be used.

The so-called ”compound-interest principle,” by which a very small charge of electricity might be employed to develop a very large one by the help of mechanical power, was first applied about a century ago in the revolving doubler. Long afterwards, Sir William Thomson availed himself of the same principle in the construction of the ”mouse-mill,”

or replenisher. The Holtz machine, the Voss and Wimshurst machines, and the other induction-machines of the same cla.s.s, all work on this principle. It may be ill.u.s.trated as follows: Take two canisters, call them A and B, and place them on gla.s.s supports. Let a very small positive charge be given to A, B remaining uncharged. Now take a bra.s.s ball, supported by a silk string. Place it inside A, and let it touch its interior surface. The ball will, as shown by Franklin, Cavendish, and Faraday, remain uncharged. Now raise it near the top of the canister, and, while there, touch it. The ball will become negatively electrified, because the small positive charge in A will attract negative electricity from the earth into the ball. Take the ball, with its negative charge, still hanging by the silk thread, and lower it into B till it touches the bottom. It will give all its charge to B, which will thus acquire a slight negative charge. Raise the ball till it is near the top of B, and then touch it with the finger or a metal rod. It will receive a positive charge from the earth because of the attraction of the negative charge on B. Now remove the ball and let it again touch the interior of A. It will give up all its charge to A; and then, repeating the whole cycle of operations, the charge carried on the ball will be greater than before, and increase in each successive operation, the electrification increasing in geometrical progression like compound interest. A Leyden jar having one coating connected to A and the other to B, may thus be highly charged in course of time. A pair of carrier b.a.l.l.s or plates, or a number of pairs, may be used instead of one. The carriers, just before leaving A and B, may be put in contact with one another instead of being put to earth; they may be mounted on a revolving shaft, and the forms of A and B modified to admit of the revolution of the carriers, and all the necessary contacts may be made automatically. We thus get various forms of the continuous electrophorus, and if the carriers are mounted on gla.s.s plates, and rows of points placed alongside the springs or brushes used for making the contacts, when the charges on the carriers become very strong, electricity will be radiated from the points on to the revolving gla.s.s plates, which will thus themselves take the place of the metal carriers. Such is the action in the Voss and other similar machines.

But after Faraday had shown how to construct a magneto-electric machine, the idea of applying the ”compound-interest principle,” and thus converting the magneto-electric machine into the ”dynamo,”

occurred apparently simultaneously and independently to Siemens, Varley, and Wheatstone. The first dynamo constructed by Wheatstone is still in the museum of King's College, London. Wilde employed a magneto-electric machine to generate a current which was used to excite the electro-magnet of a similar but larger machine, having an electro-magnet instead of a permanent steel magnet. The electro-magnet could be made much larger and stronger than the steel magnet, and from its armature, when made to revolve by steam power, a correspondingly stronger current could be maintained. The idea which occurred to Siemens, Varley, and Wheatstone was to use the whole, or a part, of the current produced by the armature to excite its own electro-magnet, and thus to dispense with the magneto-electric machine which served as the separate exciter. When a part only of the current is thus employed, and is set apart entirely for this duty, the machine is a ”shunt dynamo;” when the whole of the current traverses the field-magnet coils as well as the external circuit, it is a ”series dynamo.” The apparent difficulty lies in starting the current, but a ma.s.s of iron once magnetized always retains a certain amount of ”residual magnetism,” unless special means are taken to get rid of it, and even then the earth's magnetism would generally induce sufficient in the iron to start the action. Commencing, then, with a slight trace of residual magnetism, the revolution of the armature generates a feeble current, which pa.s.sing round the magnet coils, strengthens the magnetism, whereupon a stronger current is generated, which in turn makes the magnet still stronger, and so on until the magnet becomes saturated or the limit of power of the engine is reached, and the speed begins to diminish, or a condition of affairs is reached at which an increased current in the armature injures the magnetic field as much as the corresponding increase in the field-magnet coils strengthens it, and then no further increase of current will take place without increasing the speed of rotation. In a true dynamo the whole of the energy, both of the current and of the electro-magnets, is obtained from the source of power employed in driving the machine.

But Faraday's discovery of electro-magnetic induction led to practical developments in other directions. Graham Bell placed a thin iron disc in front of the pole of a bar magnet, and wound a coil of fine wire round the bar very near the pole. The ends of the coils of two such instruments he connected together. When the iron disc of one instrument approached the pole of the magnet, the lines of force were disturbed, fewer escaped radially from the bar, and more left it at the end, so as to go straight to the iron disc; thus the number of lines of force pa.s.sing through the coil was altered, and a current was induced which, pa.s.sing round the coil of the other instrument, strengthened or weakened its magnet, and caused the iron disc to approach it or recede from it, according to the way in which the coils were coupled. Thus the movements of the first disc were faithfully repeated by the second, and the minute vibrations set up in the disc by sound-waves were all faithfully repeated by the second instrument.

This was Graham Bell's telephone, in which the transmitter and receiver were convertible.

But another and an earlier application of Faraday's discoveries is found in the induction coil. A short length of thick wire and a very great length of thin wire are wound upon an iron bar. The ends of the long thin wire, or secondary coil, form the terminals of the machine; the short thick wire, or primary coil, is connected with a battery, but in the circuit is placed an ”interrupter.” This is generally a small piece of iron, or hammer, mounted on a steel spring opposite one end of the iron core, the spring pressing the hammer back against a screw the end of which, like the back of the hammer, is tipped with platinum; and this contact completes the battery circuit. When the current starts, the iron core becomes a magnet, attracts the hammer, breaks the contact, stops the current, the magnetism dies away, the hammer is forced back by the spring, and then the cycle of events is repeated. But the starting of the current in the primary causes a great many lines of magnetic force to pa.s.s through each of the many thousand turns of wire in the secondary, especially as the iron core conducts most of the lines of force of each turn of the primary almost from end to end of the coil, and thus through nearly all the turns of the secondary. This action might be further increased by connecting the ends of the iron core with an iron tube or series of longitudinal bars placed outside the whole coil. When the primary current ceases, all these lines of force vanish. Thus during the starting of the primary current, which, on account of self-induction, occupies a considerable time, there will be an inverse current in the secondary proportional to the rate of increase of the primary; and while the primary is dying away, there will be a direct current in the secondary proportional to its rate of decrease. The primary current cannot be increased at a faster rate than corresponds to the power of the battery, but by making a very sharp break it may be stopped very rapidly. Still, however rapidly the circuit is broken, self-induction causes a spark to fly across the gap until the energy of the current is used up. The introduction of the condenser, consisting of a number of sheets of tinfoil insulated by paper steeped in paraffin wax, and connected alternately with one end or the other of the primary coil, serves to increase the rapidity with which the primary current died away, by rapidly using up its energy in charging the condenser, and produces a corresponding diminution in the spark at the contact-breaker. This rapid destruction of the primary current causes a correspondingly great electro-motive force in the secondary coil, and thus very long sparks are produced between the terminals of the secondary coil when the primary current is broken, though no such sparks are produced when the primary current starts. If the secondary coil be connected up with a galvanometer, so that there is a metallic circuit throughout, it will be found that just as much electricity flows in one direction through the circuit at the break of the primary as flows in the other direction at the make, the difference being that the first is a very strong current of great electro-motive force but lasting a very short time, the second a feebler current lasting a correspondingly longer time.

But though the recent advances in electrical science have been very great, the grandest triumph of this century is the establishment of the principle of the conservation of energy, which has settled for ever the problem of ”the perpetual motion,” by showing that it has no solution. This problem was not simply to find a mechanism which should for ever move, but one from which energy might be continuously derived for the performance of external work--in fact, an engine which should require no fuel. But in spite of all that has been proved, numbers of patents are annually taken out for contrivances to effect this object.

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