The call-bells applied to telegraphic service have been arranged in different ways. When the vibrating bells are in use, like those of which we have just spoken, it is necessary to use a battery, and the advantages offered by telephones with induced currents are thus sensibly diminished. In order to dispense with the battery, the use of the electro-magnetic bell has been suggested.

In this case there are usually two bells, with a hammer oscillating between them, and a support formed of the polarised armature of an electro-magnet. The electro-magnetic instrument is placed below this system; it is turned by a winch, and sends the currents, alternately reversed, which are necessary to communicate the vibratory movement to the hammer, and this movement is enough to make the two bells tinkle. Below the winch of this electro-magnetic instrument there is a commutator with two contacts, which adapts the instrument for sending or receiving.

M. Mandroux has simplified this system, and has reduced it to small dimensions by the following arrangement. He fixes two magnetic cores, furnished with coils, on each of the two poles of a horseshoe magnet, composed of two bars connected by an iron coupler, and between the poles expanded by these four cores he inserts an armature, within which there is a steel spring fastened to one of these poles. In this way the armature is polarised, and oscillates under the influence of the reversed currents transmitted by an instrument of the same kind provided with an induction system. These oscillations may have the effect of producing the sound of a call-bell, and the induction system may consist of a manipulating key, fastened to a duplex system of armature, regularly applied to the magnetic cores, taken in pairs. On communicating a series of movements to this manipulator, a series of induced currents in an inverse direction are produced, which cause the armature of the corresponding station to act as we have already seen, and which may even, when necessary, furnish a series of Morse signals for a suitable manipulation. On account of the small size of this system, it might be applied to the telephonic service of the army.

The Bell Telephone Company in Paris has arranged another little call-system which is quite satisfactory and has the advantage of acting as a telephone at the same time. The model resembles the one we have termed a snuff-box telephone, and it has a button commutator by means of which the instrument is placed in communication with the electro-magnetic system of the instrument, or with a battery which is able to make the telephone vibrate with some force. To make a call, the button must be pressed, and the battery current is communicated to the corresponding instrument, which begins to vibrate when the call is made; and when notice is given of the receipt of the signal, the pressure on the button is removed, and it becomes possible to speak and receive as in ordinary telephones.

M. de Weinhold’s System.—M. Zetzche speaks highly of an alarum devised by Professor A. de Weinhold, which resembles that by M. Lorenz, represented in fig. 56. Its organ of sound consists of a steel bell T, from 13 to 14 centimètres in diameter, and toned to give about 420 double vibrations in a second. ‘Its diameter and tone,’ he says, ‘are important, and any great departure from the rule laid down diminishes the effect. The opening of the bell is below, and it is fixed on a stand by its centre. A slightly curved bar magnet, provided at its two ends with iron appendices enclosed in a coil, traverses the stand. The bar magnet of the telephone also terminates in an iron appendix enclosed in a coil. In both cases the changes produced in the magnetic condition appear to be more intense than they are in magnets without appendices. The bar magnet is placed within the bell in the direction of one of its diameters, so that the appendices almost touch its sides.

‘When the bell is struck on a spot about 90° from this diameter with a wooden clapper M, which acts with a spring, and is withdrawn by stretching the spring and then letting it go, as in a bell for the dinner-table, the vibrations imparted to it send currents into the coils, and these currents produce identical vibrations on the iron disk of the telephone, which are intensified by a conical resonator fitted to the telephone, so as to be easily heard some paces off. For ordinary use, the bell coil is broken into a short circuit by means of a metallic spring R, and consequently, when the bell is struck, the spring must be opened so as not to break the circuit. An instrument of the same kind has also been devised by Herr W. E. Fein at Stuttgardt.’

MM. Dutertre and Gouault’s System.—One of the most ingenious solutions of the problem of making the telephone call has recently been proposed by MM. Dutertre and Gouault. Figs. 57 and 58 represent the opposite faces of the instrument. It consists of a kind of snuff-box telephone, like the one shown in fig. 26, and it is so arranged as to send or receive the call, according to the way in which it is placed on its stand, which is only an ordinary bracket fastened to the wall. When it is placed on the bracket so as to have the telephone mouthpiece on the outside, it is adapted for receiving, and can then give the call. When, on the other hand, its position on the bracket is reversed, it permits the other station to make the call, by producing vibrations on a vibrator under the influence of a battery, and these vibrations reverberate in the corresponding instrument with sufficient force to produce the call. If the instrument is taken up, and the finger is placed on a small spring button, it may then be used as an ordinary telephone.

In this instrument, the magnet N S (fig. 57) is snail-shaped, like others we have mentioned, but the core of soft iron S, to which the coil E is fastened, can produce two different effects on its two extremities. On the one side, it reacts on a small armature which is fastened to the end of a vibrating disk C, fig. 58; the armature is placed against a contact fastened to the bridge B, and constitutes an electro-magnetic vibrator. For this purpose the bridge is in metallic communication with the coil wire, of which the other end corresponds with the line wire, and the spring C is mounted on an upright A, which also supports another spring D G acting on two contacts, one placed at G, and corresponding to the earth wire, the other at H, and connected with the positive pole of the battery. A small moveable button, which passes through a hole in the lid of the box, and projects beyond it, is fixed at G, and all this part of the instrument faces the bottom of the box. The upper part consists of the vibrating disk and the mouthpiece, so that the mechanism we have described is all mounted on an inner partition forming a false bottom to the box.

When the box rests upon its base, on the side shown in fig. 58, the button at G presses on the spring D G, and raises it so as to break the connection with the battery; the coil of the instrument is then united to the circuit, and consequently receives the transmitted currents, which follow this route: line wire, coil E, bridge B, spring C, spring D G, earth contact. If these currents are transmitted by a vibrator, they are strong enough to produce a noise which can be heard in all parts of a room, and consequently the call may be given in this way. If the currents are due to telephonic transmission, the instrument is applied to the ear, care being taken to put the finger on the button G, and the exchange of correspondence takes place as in ordinary instruments; but it is simpler and more manageable to insert a second telephone in the circuit for this purpose. When the box is inverted on its mouthpiece, and the button G ceases to press on the spring D G, the battery current reacts on the vibrator of the instrument, and sends the call to the corresponding station, following this route: I D A C B E, line, earth and battery; and the call goes on until the correspondent breaks the current by taking up his instrument, thus warning the other that he is ready to listen.

System of M. Puluj.—There is yet another call system, devised by M. Puluj. It consists of two telephones without mouthpieces, connected together, and with coils placed opposite the branches of two tuning-forks, tuned as nearly as possible to the same tone. A small metal bell is fixed between the opposite faces of the tuning-forks, and a wire stretched near them is provided with a small ball in contact with their branches. When the tuning-fork at the sending station is put in vibration by striking it with an iron hammer covered with skin, the tuning fork at the other station vibrates also, and its ball strikes upon the bell. As soon as the signal is returned by the second station, mouthpieces with iron diaphragms are fastened to the telephones, and the correspondence begins. It seems that, by the use of a resonator, the sound which reaches the receiving station may be so intensified as to become audible in a large hall, and the bell signal may be heard in an adjoining room, even through a closed door.

Mr. Alfred Chiddey’s System.—This arrangement consists of a slender copper tube, eight inches long, and with an orifice of 1/30 of an inch, of which the lower end is soldered to the diaphragm of a telephone. A branch joint, to which an india-rubber tube is fitted, connects it with a gas jet, which is lighted and surrounded with a lamp shade, in such a way as to make it produce, under given conditions, sounds resembling those of the singing flames. A perfectly similar system is arranged at the other end of the line, in such a way that the sounds emitted in each case shall be precisely in unison. If the two systems are so regulated as not to emit sounds in their normal condition, they can be made to sing by causing a tuning-fork in the vicinity of one or the other to vibrate the same note, and then the corresponding flame will begin to sing, producing a vibration in the diaphragm of the telephone with which it is in correspondence, and hence will follow the vibration of the diaphragm of the other telephone, and consequently the vibration of the flame of the calling instrument. In this way the call signal may be made without the intervention of any battery.