EXAMINATION INTO THE FUNDAMENTAL PRINCIPLES ON WHICH BELL’S TELEPHONE IS BASED.
Although the preceding account would suffice to make the principle of Bell’s telephone intelligible to persons acquainted with electric science, this would not be the case with the majority of our readers, and we therefore think it necessary to enter into some details as to the source of the electric currents which are employed in telephonic transmissions. These details seem to us the more necessary, since many persons still believe that Bell’s telephones are not electric, because they do not require a battery, and they are often confounded with string telephones, so that the difference of price between Bell’s instruments and those hawked in the streets seems astonishing.
Without defining what is meant by an electric current, which would be too elementary, we may say that electric currents can be produced by different causes, and that, in addition to those which are due to batteries, strong currents are also produced by the force exerted by magnets on a conducting circuit properly arranged. Such currents are called induction currents, and are used in Bell’s telephone. In order to understand how they are developed under these conditions, it will be enough to examine what takes place when the pole of a magnet is brought near to, and withdrawn from, a closed circuit. To do this, let us suppose a copper wire attached to a galvanometer in the form of a circle, and that one pole of a permanent magnet is directed towards the centre of the circle. Now observe what happens:
1. At the moment when the magnet approaches an electric current arises, causing the galvanometer to deviate to one side. This deviation will be great in proportion to the extent of the movement, and the tension of the current will be great in proportion to the abruptness of the movement. The current will however be only instantaneous.
2. At the moment when the magnet is withdrawn, a fresh current of the same nature will arise, but it will appear in an opposite direction from the former. It will be what is called a direct current, because it is in the same direction as the magnetic current of the magnet which produces it, while the other current is called inverse.
3. If, instead of advancing or withdrawing the magnet by means of a single movement, it is advanced in jerks, a succession of currents in the same direction is produced, of which the existence can be ascertained by the galvanometer when there is a sufficient interval between the movements, but when the intervals are very slight the currents are interfused; and since inverse effects take place when the magnet is moved in a contrary direction, the needle of the galvanometer follows the movements of the magnet, and to a certain extent stereotypes them.
4. If, instead of reacting on a simple closed circuit, the magnet exerts its force on a considerable number of circumvolutions of this circuit, that is, on a bobbin of coiled wire, the effects will be considerably increased, and they will be still greater if there be a magnetic core within the bobbin, since the inducing action will then be more effectually exerted throughout the bobbin. As the magnetic core, when it is magnetised and demagnetised under the influence of its approach to or withdrawal from the inducing magnet, is subject to the reaction from all the fluctuations which occur in the movements of the magnet, the induced currents which ensue are perfectly defined.
5. If, instead of a movable magnet, we suppose it to be fixed in the centre of the coil, the induced currents of which we have spoken may then be determined by modifying its force. In order to do so, it is enough that an iron armature should react upon its poles. When this armature is brought close to one of the poles, or to both at once, it acquires force, and produces an inverse current, that is, a current in the direction which would have corresponded to an approach of the magnet to the closed circuit. On its withdrawal the inverse effect is produced; but in both cases the induced currents correspond with the extent and direction of the movements accomplished by the armature, and consequently they may reproduce its movements by their effects. If this armature is an iron plate, which vibrates under the influence of any sound in this disposition of the electro-magnetic system, the alternate movements of the plate will be transformed into the induced currents, and these will be stronger or weaker, more or less definite, according to the range and complexity of the vibrations: they will, however, be undulatory, since they will always result from successive and continuous movements, and will consequently be in the conditions which, as we have seen, are required for the transmission of speech.
As for the action produced upon the receiver, that is, on the instrument for reproducing speech, it is somewhat complex, and we shall have occasion to speak of it presently; but we can get a general impression of it, if we consider that the effects produced by the induced currents of variable intensity, which traverse the coil of the electro-magnetic system, must determine, by the magnetisations and demagnetisations which ensue, the vibrations of the armature disk; these vibrations, more or less amplified and defined, exactly represent those of the disk before which the speaker stands, and can only be obtained from them. The effects are, however, in reality more complex, although they are produced under analogous conditions, and we shall have more to say about them when we come to speak of the experiments made with the telephone. It must meanwhile be observed that, for the reproduction of speech, it is not necessary that the magnetic core should be of soft iron, since the vibratory effects may follow from differential as well as from direct magnetisation.