CHAPTER X. TRANSFORMATIONS OF ENERGY. I HAVE dwelt upon the construction of a galvanometer and of the dynamo machine in order to emphasize the great bearing that properly constructed machines have upon the progress of science. Faraday's galvanometer was not a very sensitive instrument; it was analogous to the one-lens microscope of Leuwenhoek; yet it was sufficient to show the great law of magneto-induction, that the movement of a coil of wire near a magnetic pole produces a current of electricity in the coil, and when it was aided by powerful auxiliaries it showed that any change in an electric current produced a current of electricity through the ether of space in neighbouring conductors. These powerful helps consisted of a strong voltaic battery of large plates, and a great many of them, and of a powerful electro-magnet which Joseph Henry had shown how to construct and how to use with the battery in order to obtain the greatest effect. Faraday made his principal discoveries in magneto-induction in ten days when he was at the age of forty-two. There is little doubt in my mind that other men would speedily have discovered the same phenomena, for their universality could not long have eluded observation. Faraday's great achievement was in his conception of the lines of force which emanate from a magnetic pole and stretch through the ether of space; in his pointing out that the medium surrounding the wires carrying electric currents, and the medium in which magnets are situated, is in a state of strain; that there is what he called an electrotonic state of this medium. It was like a mass of quivering jelly-any movement at one point produced a quiver in all neighbouring points. Maxwell, in his great work on electricity, thus speaks of Faraday's conception: "Faraday saw lines of force traversing all space where the mathematicians saw centres of force attracting at a distance. Faraday sought the seat of the phenomena in real actions going on in the medium; they were satisfied that they had found it in a power of action at a distance impressed on the electric fluids."* One obtains a realizing sense of the intellectual power of Faraday by reading his Experimental Researches in Electricity. He is not merely an inventor who, having discovered some phenomenon of Nature, proceeds to put it to a practical use; but each experiment is guided by a remarkable generalizing faculty, and the series of his experimental researches led him to the conception of electrical actions in the medium of space and laid the foundation of the greatest generalization in science of modern times-Maxwell's electromagnetic theory of light. We shall be led to this great theory as we continue our study of the question, "What is electricity?" Meanwhile let us examine a little further the phenomena discovered by Faraday. A delicate galvanometer reveals, we have seen, that the motion of a wire near a magnet results in an electric current in the wire. Why this is so we do not * Preface to Maxwell's Electricity and Magnetism. know. We have We have a good working theory, however, which we shall express later. Since the earth is a magnet, any motion of a wire on its surface will cause a current in the wire. One can signal under the sea through a cable by properly waving a coil of wire in the air. We say that the resulting current of electricity is due to cutting the lines of magnetic force of the earth by the motion of the wire. For forty years, nearly half a century after Faraday's great discovery of magneto-induction, men's minds were almost exclusively devoted to obtaining steady currents of electricity in one direction instead of to-and-fro currents, such as are obtained by rapidly thrusting a north pole into a spool of wire and rapidly withdrawing it. The commutator was improved in every possible way until the commuting of the directions of the to-and-fro currents had well-nigh become perfect. The commutator in the best forms of the modern dynamo machine shows very little sparking, whereas in the earlier forms there was a brilliant coruscation of sparks when the segments of the commutator ran under the brushes which collected the current for the outer circuit in which the electrical work was to be done. These sparks showed that energy was lost. There is very little room for improvement at present in the modern dynamo. It approaches nearer to perfection than any other machine which is used to transmit power. It is doubtful in my mind whether Faraday ever realized the powerful effects that could be obtained when his lines of magnetic force were made to quiver with great speed. The galvanometer employed by him could only detect steady currents, or momentary currents which reversed in direction very slowly. It was incapable of showing any effect when the currents of induction were sent to and fro through it very rapidly. It would remain perfectly quiescent, its little needles pointing placidly north and south, while to-and-fro currents of tremendous energy were circulating through the circuit with which it was connected. It was like a deaf-mute with respect to the world of harmonies of an orchestra. Although these to-and-fro currents of electricity annulled each other's effect on the needle of the galvanometer, they could produce an electric light, could heat wires, and, in short, produce all the effects obtained from steady currents with the exception that they could not run an electric motor of the type which we have considered the type which had been slowly perfected during the forty years after Faraday's discoveries. We are now entering upon another period of electrical invention which may be called the period of adaptation of Faraday's discoveries to instruments adapted to toand-fro currents instead of steady currents, and we shall see the necessity of using to-and-fro currents instead of steady currents for the transmission of electrical power, as we continue our study. I have said that it is questionable in my mind whether Faraday realized the wonderful development which is now beginning in the commercial employment of to-and-fro currents. certainly, however, had a full conception of the sensitiveness of the electrotonic state of the medium surrounding magnets and electrical circuits, but he had no instruments which could represent to other people's eyes and ears the wonders of his imagination. He The telephone is an instrument based entirely upon Faraday's discovery of magneto-induction. If we should take the spools which we have used (Fig. 10) to illustrate the action of a commutator, slip each of them upon the poles of a magnet, as in Fig. 12, connect the ends of the wires of the spools permanently, place a thin disk of iron, such as is used in taking tintypes in photography, very near to each pole, providing a suitable earpiece to each telephone, we would find that a mere tap with the finger on the iron disk of telephone A, for instance, can be heard, on listening at that of telephone B, even when the telephone B is at the distance of many miles from telephone A. The operation of this tap is like thrusting rapidly a magnetic pole into the spool of B and quickly withdrawing it to-and-fro currents of induction are produced which attract and repel the disk of the telephone B, thus reproducing the mechanical action which is exerted by tapping the instrument A. It is more correct as well as more picturesque to say that the quivering of the magnetic lines of force due to the slight movement of the thin iron disks affects the electrotonic state of the medium in which the telephones are immersed. The telephone into which we speak corresponds to the electric dynamo, and the telephone to which we listen to the electric motor. But here there is no commutator; we are using to-and-fro currents, or, in ordinary prac |