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. Œrsted'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 illustration 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 pass 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 passed 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 circumference, or vice versâ, 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 circumference, or in the reverse direction, in that part of the disc which was passing 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 spaces 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 pass through the coils in certain positions than would pass 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 class, all work on this principle. It may be illustrated as follows: Take two canisters, call them A and B, and place them on glass supports. Let a very small positive charge be given to A, B remaining uncharged. Now take a brass 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 balls 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 glass 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 glass 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 mass 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 passing 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 passing through the coil was altered, and a current was induced which, passing 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 pass 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.
We have seen how Rumford showed that heat was motion, and how he approximately determined its mechanical equivalent. Séguin, a nephew of Montgolfier, endeavoured to show that, when a steam-engine was working, less heat entered the condenser than when the same amount of steam was blown idly through the engine. This Hirn succeeded in showing, thus proving that heat was actually used up in doing work. Mayer, of Heilbronn, measured the work done in compressing air, and the heat generated by the compression, and assumed that the whole of the work done in the compression, and no more, was converted into the heat developed, which was the same thing as assuming that no work was done in altering the positions of the particles of gas. From these measurements he deduced a value of the mechanical equivalent of heat. The assumption which Mayer made was shown experimentally by Joule to be nearly correct. Joule proved that, when air expands from a high pressure into a vacuum, no heat is generated or absorbed on the whole. This he did by compressing air in an iron bottle, which was connected with another bottle from which the air had been exhausted, the connecting tube being closed by a stop-cock. The whole apparatus was immersed in a bath of water, and on allowing the air to rush from one vessel into the other, and then stirring the water, the temperature was found to be the same as before. When the iron bottles were in separate baths of water, that from which the air rushed was cooled, and that into which it rushed was heated to the same extent. Joule and Thomson afterwards showed that a very small amount of heat is absorbed in this experiment. Joule also showed that the heat generated in a battery circuit is proportional to the product of the electro-motive force and the current, or to the product of the resistance and the square of the current, which, in virtue of Ohm's law, is the same thing. This relation is often known as Joule's law. He also proved that, for the same amount of chemical action in the battery, the heat generated was the same, whether it were all generated within the battery or part in the battery and part in an external wire; and that in the latter case, if the wire became so hot as to emit light, the heat measured was less than before, on account of the energy radiated as light. With a magneto-electric machine he employed mechanical power to produce a current, and the energy of the current he converted into heat. In all cases he found that, whatever transformations the energy might undergo in its course, a definite amount of mechanical energy, if entirely converted into heat, always produced the same amount of heat; and he thereby proved, not only that heat is essentially motion, but that it corresponds precisely with that particular dynamical quantity which is called energy; and thus justified the attempt to find a relation between heat and energy, or to express the mechanical equivalent of heat as so many foot-pounds.