The arrangement most generally adopted for the telephone is the one represented in fig. 21. It consists of a kind of circular wooden box, fitted to the extremity of a handle M, which is also of wood, and contains the magnetic bar N S. This bar is fixed by means of a screw t, and is so arranged as to be moved forward and backward by tightening or loosening the screw, a condition necessary in order to regulate the instrument. At the free extremity of the bar the magnetic coil B is fixed; this must, according to MM. Pollard and Garnier, be made of wire No. 42, so as to present a considerable number of spirals. The ends of this coil generally terminate at the lower end of the handle in two copper rods f, f, which traverse its length, and are fastened to two binding-screws I, I′, where the line wires C, C are fixed. In the instruments made by M. Bréguet there are, however, no binding-screws, but a little twist, made of two flexible wires covered with gutta-percha and silk, is fastened to the two rods. A wooden cap is screwed to the end of the handle, and the twist passes through a hole made in this cap, so that there is no inconvenience in working the instrument. By laying hold of the ends of the wire twist with pliers it is possible to join them to the circuit. This instrument is represented in fig. 22.
By another arrangement, the wires of the coil end immediately in the binding-screws which are placed below the wooden box, but this arrangement is inconvenient.
Above the pole of the magnetic bar is placed the iron vibrating plate L L, which is coated either with black or yellow varnish, with tin or blue oxide, but which must always be very thin. This plate is in the form of a disk, and by its rim, resting on a caoutchouc ring, it is firmly fixed to the circular edges of the wooden box, which is for this purpose made in two pieces. These pieces are adjusted to each other, either by screws or by spirals cut in half the thickness of the wood. This disk ought to be as near as possible to the polar end of the magnet, yet not so near as to produce contact between the two by the vibrations of the voice. Finally, the mouthpiece R R′ (fig. 21), which is in form of a wide funnel, terminates the upper part of the box, and should be so arranged as to leave a certain space between the disk and the edges of the hole V, which is open in its centre. The size of the box should be so calculated as to permit of its acting as a sounding-box, without however provoking echoes and a confusion of sounds.
When the instrument is properly made, it will produce very marked effects; and M. Pollard, one of the first Frenchmen to take up the study of telephones, has written as follows on the subject:
‘The instrument which I have prepared gives results which are truly astonishing. In the first place, when considering the resistance, the introduction into the circuit of five or six persons does not sensibly diminish the intensity of sounds. On putting an instrument to each ear, the sensation is precisely the same as if the correspondent were speaking some yards behind. The intensity, the clearness, the purity of tone are irreproachable.
‘I can speak to my colleague in quite an undertone, scarcely breathing as I may say, and persons placed within two yards of me will be unable to catch a single word of our conversation.
‘On the part of the receiver, if anyone raises his voice to call me, I hear the call in all parts of my office, at least when silence prevails there; at any rate, when I am seated at my table with the instrument some yards off, I can always hear the call. In order to increase the intensity of sound, I fitted the mouthpiece with a copper horn of conical shape, and under these conditions words spoken in my bureau two or three yards from the mouthpiece can be heard at the other end of the line; from my station, a little more than a yard from the tube, I can hear and speak to my colleague without effort.’
In using the ordinary Bell telephone, it is necessary to speak distinctly before the mouthpiece of the telephone which is handled, while the listener placed at the corresponding station keeps the mouthpiece of the receiver to his ear. These two instruments form a closed circuit with the two wires which connect them, but one is enough to make the transmission perfect, if care is taken to place both instruments in connection with the earth, which thus takes the place of the second wire. M. Bourbouze asserts that the intensity of sound in the telephone is much increased by employing this expedient, but we believe that this increase depends upon the conditions of the circuit, although he asserts that the fact can be proved in a circuit not exceeding eighty yards.
For practical purposes it is necessary to have two telephones at each station, so as to hold one to the ear while speaking through the other, as in fig. 23. It is also much more easy to hear with a telephone applied to each ear, in which case they are held as in fig. 24. In order not to fatigue the arms, an arrangement has been made by which they are held before the ears by a strap and spring which goes round the head.
The sending power of the telephone varies with different voices. Mr. Preece asserts that shouting has no effect, and that, in order to obtain a favourable result, the intonation must be clear, the articulation distinct, and the sounds emitted must resemble musical sounds as much as possible.
Mr. Wilmot, one of the electricians employed by the Post Office, says that he has been able to make himself heard on circuits through which no other voices were audible. The vowel sounds are most readily transmitted, and among other letters e, g, j, k, and q are always repeated more imperfectly. The ear requires practice, and the faculty of hearing varies in a surprising degree in different people. Singing is very distinctly heard, as well as wind instruments, especially the cornet-à-piston, which, when played in London, was heard by thousands of people in the Corn Exchange at Basingstoke.
According to Mr. Rollo Russell, it is not necessary to isolate the circuit of a telephone when the distance is relatively slight; thus, with a circuit of about 430 yards, it is possible to use a simple copper wire, laid on the grass, without destroying the telephonic transmission from a small musical box, as long as the two wires do not touch each other. Transmission took place, even when the circuit was buried in moist earth for a length of thirty-five yards, or immersed in a well for a length of forty-eight yards. The words transmitted under such conditions did not differ from those transmitted by an isolated circuit.
The telephone may be heard at the same moment by several listeners, either by connecting the wires which unite the telephones in correspondence (near the receiving telephone) with branch wires of other telephones, which may be done up to the number of five or six, in short circuits; or by means of a little sounding-box closed by two thin membranes, one of which is fixed on the vibrating disk. When a certain number of acoustic tubes are connected with the membrane, Mr. M’Kendrick asserts that several people can hear distinctly.
Telephones may also transmit speech to different stations simultaneously, by inserting them on the same circuit, and experiments made at New York showed that five instruments placed in different parts of the same telegraphic line could be made to speak in this way. In the telephonic experiments made on the canal lines in the department of the Yonne, it was ascertained that on a wire seven miles and a half in length, on which several telephones were placed at varying distances, three or four persons were able to converse with each other through the telephones, and each could hear what the other was saying. The questions and answers could be understood, even in crossing. It was also possible, by placing a telephone on a second wire, a little over five miles in length, and half a yard distant from the other, to hear the conversation exchanged on the first wire by following it to a distance not exceeding a mile and a quarter. Even the different voices of the two speakers could be distinguished.
Since the telephone made its appearance in Europe, several inventors have asserted that they are able to make a telephone speak so as to be audible in all parts of a large hall. It has been shown that this was accomplished by Mr. Bell, and in this respect we do not see that those who have attempted to improve the telephone have attained results of greater importance. It is certain that the ordinary telephone can emit musical sounds which become perfectly audible in a tolerably large room, while the instrument is still attached to the wall. We should also remember the results obtained by MM. Pollard and Garnier in the experiments made at Cherbourg to connect the mole with the Préfecture Maritime.
The mole at Cherbourg is, as we know, a kind of artificial island thrown up before the town in order to make an anchorage. The forts which have been constructed on the mole are connected by submarine cables with the military port and with the Préfecture Maritime. On one occasion, after making experiments in the Préfet’s study on one of the cables applied to a telephone, several persons were talking together in the room, and were much surprised to hear the bugle sound the retreat, the sound appearing to come from one part of the room. It was found, on examination, that the telephone hung to the wall was occupied with this performance. On enquiry, it appeared that one of the manipulators on the mole station had amused himself by sounding the bugle before the telephone on that station. The mole is more than three miles from Cherbourg, and the Préfecture Maritime is in the centre of the town. Yet these telephones had been roughly made in the dockyard workshops; and we have here another proof of the small amount of accuracy required for the successful working of these instruments.
Telephones of various construction on the Bell model are to be seen at M. C. Roosevelt’s, Mr. Bell’s agent in Paris, 1, Rue de la Bourse. They are, for the most part, constructed by M. Bréguet, and the model in the greatest request, exclusive of the one we have described, is the great square model, with a horseshoe magnet enclosed in a flat box, and a horn on its upper side, which serves as a mouthpiece. This system is represented in fig. 25, and it has been neatly constructed at Boston under the best conditions. In this new model, made by Mr. Gower, the magnet is composed of several plates terminated by magnetic cores of iron, to which the coils are fixed, and the whole is covered with a thick layer of paraffin. The sounds thus reproduced are much stronger and more distinct. Mr. Gower, who is now Mr. Roosevelt’s partner, has made considerable improvements in the different forms of Mr. Bell’s instrument. There is one model in the form of a snuff-box, in which the magnet is twisted into a spiral, so as to maintain its length in a circular form. The pole, which is in the centre of the spiral, is furnished with an iron core, to which the induction coil is fastened, and the cover of the snuff-box supports the vibrating disk as well as the mouthpiece: this model is represented in fig. 26. In another model, called the mirror telephone, the preceding arrangement is fitted on to a handle like the glass of a portable mirror, and there is a mouthpiece on one of the lateral faces, so that the speaker uses the instrument as if he were speaking before a chimney screen.
Mr. Bailey has different models of telephones worked by a battery or by the Edison carbon of which we shall speak presently, and these, as well as the telephones by Messrs. Gray and Phelps, are more successful in conveying sound on a long line of wire.
The prodigious results attained with the Bell telephones, which were at first discredited by many scientific men, necessarily provoked, as soon as their authenticity was proved, innumerable researches on the part of inventors, and even of those who were originally the most incredulous. A host of improvements and modifications have consequently been suggested, which are evidently not without interest, and must now be considered by us.