DIFFERENT ARRANGEMENTS OF TELEPHONES. 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. BATTERY TELEPHONES. _The Edison Telephone._--One of the earliest and most interesting improvements made in the Bell telephone is that introduced by Mr. Edison in the early part of the year 1876. This system is indeed more complicated than the one we have just considered, since it requires a battery, and the sending instrument differs from the receiving instrument; but it is less apt to be affected by external causes, and transmits sound to a greater distance. The Edison telephone, like Mr. Gray’s, which we have already had occasion to mention, is based upon the action of undulatory currents, determined by the variations in the resistance of a conductor of moderate conducting power, which is inserted in the circuit, and the vibrations of a diaphragm before which the speaker stands react upon it. Only, instead of employing a liquid conductor, which is practically useless, Mr. Edison has attempted to use semi-conducting solid bodies. Those which were most suitable from this point of view were graphite and carbon, especially the carbon extracted from compressed lamp-black. When these substances are introduced into a circuit between two conducting plates, one of which is moveable, they are capable of modifying the resistance of the circuit almost in the same proportion as the pressure exerted upon them by the moveable plate,[5] and it was seen that, in order to obtain the undulatory currents necessary for the production of articulate sounds, it was enough to introduce a disk of plumbago or of lamp-black between the vibrating plate of a telephone and a platinum plate placed in connection with the battery. When the telephone disk is placed in circuit, its vibrations before the disk of carbon produce a series of increasing and decreasing pressures, thus causing corresponding effects in the intensity of the transmitted current, and these effects react in an analogous manner on the undulatory currents determined by induction in the Bell system. In order to obtain good results, however, several accessory arrangements were necessary, and we represent in fig. 27 one of the arrangements made in this part of Mr. Edison’s telephonic system. [Illustration: FIG. 27.]