1st. To the telephone circuit. 2nd. To that of the bell system. 3rd. To that of the local battery. The second fork, which supports the telephone on the right, is fixed to the board, and is independent of any electric current. It is clear that this arrangement may be varied in a thousand ways, but the model we have just described is the most practical. _Edison’s System._--The problem becomes more complex in the case of battery telephones, since the battery must be common to both systems, and the induction coil must be inserted in two distinct circuits. Fig. 55 represents the model adopted in Mr. Edison’s telephone. [Illustration: FIG. 55.] In this arrangement, there is a small stand C on the mahogany board on which the bases of the two telephones rest. The bell system S is worked by an electro-magnetic speaker P, which serves, when a Morse key is added to the system, for exchange of correspondence in the Morse code, if there should be any defect in the telephones, or to put them in working order. Above the speaker there is a commutator with a stopper D to adapt the line for sending or receiving, with or without the bell; and below the stand C the induction coil, destined to transform the voltaic currents into induced currents, is arranged in a small closed box E. When the commutator is at reception, the line is in immediate correspondence either with the speaker or with the receiving telephone, according to the hole in which the stopper is inserted; when, on the other hand, it is at sending, the line corresponds to the secondary circuit of the induction coil. Under these conditions the action is no longer automatic; but since this kind of telephone can only be usefully employed for telegraphy, in which case those who work it are acquainted with electric apparatus, there is no inconvenience in this complication. CALL-BELLS AND ALARUMS. 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.