/')c// Tc/cprone.—The practical introduction of the art of telephony unquestionably dates from 1876, when Alexander Graham Bell, an Amer ican inventor, displayed his articulating telephone (fig. 7) at the Centennial Exhibition held in Philadelphia.
Extension and Improvement of the its introduction the telephone has become one of the necessities of modern civilization. At first the imperfection of the apparatus and the methods employed limited its use to comparatively short intervals, but at present telephone-lines are working satisfactorily over distances of several hundred miles, and the in habitants of cities such as Philadelphia, New York, Boston, etc., may by this means converse with one another without difficulty. This of itself suffices to indicate the perfection to which the art of telephony has been brough t.
The details of the system will be found fully eluci dated in the volume on Physics, to which the reader is accordingly referred. It may, however, suffice here to indicate in a general way the three essen tial elements of the method—namely, the telephone, the transmitter, and the exchange-table. Fig-ures 6 and 7 exhibit, respectively, the receiving apparatus and the transmitting- apparatus. The telephone (fig. 6) consists of a thin metallic diaphragm (E) suitably fixed in a wooden frame (F), and having a bar of magnetized iron secured in the stem of the instrument at right angles to the diaphrag-in (A), and in such a manner that it may be adjusted. The magnetized bar is never in actual contact with the dia phragm. The bar is, furthermore, surrounded by a coil of insulated wire at the extremity next to the diaphragm (B), and this coil is connected both with the line wire and with the ground (C, D).
Principle of sounds are thrown into the mouthpiece of the instrument, they cause vibrations of the diaphragm, and, as the dia phragm—which is formed of a thin flexible sheet of iron—is made by these vibrations to approach nearer to and recede farther from the magnetized bar, fluctuations in the magnetism of the bar are thereby produced. These
fluctuations are reproduced by induction in the coil surrounding the mag net, and by this are transmitted over the line wire to the other terminus, where they cause exactly similar vibrations in the diaphragm of the receiv ing apparatus, and where, by a reversal of the process above described, there are produced the same sounds that were projected into the apparatus at the transmitting end. The process, then, consists in the conversion of the sound-waves, in the first instance, into electrical waves, the transmis sion of these over the line wire, and the reconversion of these electrical waves at the other end of the line into sound-waves of the same character as those projected into the transmitting end. Thus articulate speech is conveyed over g-reat distances and intelligibly reproduced with astonishing accuracy. In actual practice the receiving apparatus is a modification of that just described, but the principle of its operation is substantially the same. .A. detailed description of the various parts of the devices employed would too greatly extend this account.
The Telephone Exchange is a system by which the participators in a , local telephone service are brought into communication with one another at will through the intervention of a central office, or exchange, with which the wires of each subscriber are connected. When one wishes to converse with any other, the central office, being signalled for the purpose and told the number wanted, places the caller in telephonic communication with the desired subscriber.
The Telephone, by which acoustic communication is maintained between places one hundred, and even two hundred, miles apart, is operated on substantially the same principles, save that in this system it is found necessary to operate with a complete return metallic circuit, or double line.