Now, by placing the spiral in front of spiral C, the telephone reproduces the sound given out by the tuning-fork so loudly that I have no doubt all of you can hear it. Here is another spiral similar in every respect to spiral C. This is in circuit with a battery and an ordinary mechanical make and break arrangement, the sound given off by which I will now make audible to you in the same way that I did the sound of the tuning-fork. Now you hear it. I will change from the one spiral to the other several times, as I want to make you acquainted with the sounds of both, so that you will have no difficulty in distinguishing them, the one from the other.
There are suspended in this room self-luminous bodies which enable us by their rays or lines of force to see the non-luminous bodies with which we are surrounded. There are also radiating in all directions from me while speaking lines of force or sound waves which affect more or less each one of you. But there are also in addition to, and quite independent of, the lines of force just mentioned, magnetic lines of force which are too subtle to be recognized by human beings, consequently, figuratively, we are both blind and deaf to them. However, they can be made manifest either by their notion on a suspended magnet or on a conducting body moving across them; the former showing its results by attraction and repulsion, the latter by the production of an electric current. For instance, by connecting the small flat spiral of copper wire in direct circuit with the galvanometer, you will perceive that the slightest movement of the spiral generates a current of sufficient strength to very sensibly affect the galvanometer; and as you observe, the amplitude of the deflection depends upon the speed and direction in which the spiral is moved.
We know that by moving a conductor of electricity in a magnetic field we are able to produce an electric current of sufficient intensity to produce light resembling in all its phases that of solar light; but to produce these strong currents, very powerful artificial magnetic fields have to be generated, and the conductor has to be moved therein at a great expenditure of heat energy. May not the time arrive when we shall no longer require these artificial and costly means, but have learned how to adopt those forces of nature which we now so much neglect? One ampere of current passing through an ordinary incandescent lamp will produce a light equal to ten candles, and I have shown that by simply moving this small flat spiral a current is induced in it from the earth's magnetic field equal to 0.0007 ampere. With these facts before us, surely it would not be boldness to predict that a time may arrive when the energy of the wind or tide will be employed to produce from the magnetic lines of force given out by the earth's magnetism electrical currents far surpassing anything we have yet seen or of which we have heard.
Therefore let us not despise the smallness of the force, but rather consider it an element of power from which might arise conditions far higher in degree, and which we might not recognize as the same as this developed in its incipient stage.
If the galvanometer be replaced by a telephone, no matter how the spiral be moved, no sound will be heard, simply because the induced currents produced consist of comparatively slow undulations, and not of sharp variations suitable for a telephone. But by placing in circuit this mechanical make and break arrangement the interruptions of the current are at once audible, and by regulating the movement of the spiral I can send signals, which, if they had been prearranged, might have enabled us to communicate intelligence to each other by means of the earth's magnetism. I show this experiment more with a view to illustrate the fact that for experiments on induction both instruments are necessary, as each makes manifest those currents adapted to itself.
The lines of force of light, heat, and sound can be artificially produced and intensified, and the more intense--they are the more we perceive their effects on our eyes, ears, or bodies. But it is not so with the lines of magnetic force, for it matters not how much their power is increased--they appear in no way to affect us. Their presence can, however, be made manifest to our eyes or ears by mechanical appliances. I have already shown you how this can be done by means of either a galvanometer or a telephone in circuit with a spiral wire.
I have already stated that while engaged in these experiments I found that as far as the telephone was concerned it was immaterial whether it was in circuit with a spiral or not, as in either case it accurately reproduced the same sounds; therefore, much in the same way as lenses assist the sight or tubes the hearing, so does the telephone make manifest the lines of intermittent inductive energy. This was quite a new phenomenon to me, and on further investigation of the subject I found that it was not necessary to have even a telephone, for by simply holding a piece of iron to my ear and placing it close to the center of the spiral I could distinctly hear the same sounds as with the telephone, although not so loud. The intensity of the sound was greatly increased when the iron was placed in a magnetic field. Here is a small disk of iron similar to those used in telephones, firmly secured in this brass frame; this is a small permanent bar magnet, the marked end of which is fixed very closely to, but not touching, the center of the iron disk. Now, by applying the disk to my ear I can hear the same sounds that were audible to all of you when the telephone in circuit with a small spiral was placed in front of and close to the large spiral.
To me the sound is quite as loud as when you heard it; but now you are one and all totally deaf to it. My original object in constructing two large spirals was to ascertain whether the inductive lines of force given out from one source would in any way interfere with those proceeding from another source. By the aid of this simple iron disk and magnet it can be ascertained that they do in no way interfere with each other; therefore, the direction of the lines proceeding from each spiral can be distinctly traced. For when the two spirals are placed parallel to each other at a distance of 3 ft. apart, and connected to independent batteries and transmitters, as shown in Plate 7, each transmitter having a sound perfectly distinct from that of the other, when the circuits are completed the separate sounds given out by the two transmitters can be distinctly heard at the same time by the aid of a telephone; but, by placing the telephone in a position neutral to one of the spirals, then only the sound proceeding from the other can be heard. These results occur in whatever position the spirals are placed relatively to each other, thus proving that there is no interference with or blending of the separate lines of force.