cities have been most marked, and this form of apparatus is now so complex as to be quite unintelligible except to a telephone engineer, as almost every part of the apparatus has been subjected to important improvements.
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| TELEPHONE CIRCUIT WITH HOOK DOWN AND WITH HOOK UP. |
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a and c binding posts connecting with the line. The post b forms the ground connection for lightning. The dotted lines indicate that part of the circuit cut out by the hook switch. |
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| DIAGRAM OF SIMPLE METALLIC CIRCUIT SWITCHBOARD. |
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When a call is received the current coming in on the line wires causes the drop to fall, thus announcing the number of the subscriber. The central operator then inserts the answering plug in the jack, and, pressing the listening key, puts her instrument in circuit. She then inserts the calling plug in the jack belonging to the line of the desired subscriber, and by pressing the ringing key sends a signal to the latter's instrument. |
The use of a common battery at the central station has been one of the most important developments of recent years, as it does away with the magneto call at the subscriber's instrument, a saving of no small dimensions, as the magneto machine was the most expensive part of the equipment, the mere removal of the receiver informing the central operator of the subscriber's presence at the telephone. In the Hayes system, which is extensively employed in the United States, a device known as a repeating coil is used, and the battery is bridged across the circuit at the central station. The repeating coil consists of a transformer or induction coil formed by two coils of wire of equal length and size, it being customary to wind the four coils required for two repeating coils on the same core. There are two sets of windings in each circuit and the current of the battery divides and passes through a single coil before reaching the line or main circuit wires. In other words, the negative pole of the battery connects with the two sets of windings of one repeating coil, one of which joins the wires leading to each station, while the positive pole is similarly connected on the other side. Any variation in the current caused by the action of the sound waves on the transmitter of one circuit will produce similar effects in the other circuit by inductive action. Such is the case when the line is arranged for talking. To signal the central station the subscriber removes his receiver from the hook, thus closing a circuit which acts upon a relay and causes a small incandescent lamp to glow. This gives the signal to the central operator, who immediately completes the talking circuit first with her own transmitter and receiver, and then with that of the desired subscriber. The attention of the latter has been attracted by a call bell energized by an alternating current. This bell is in series with a condenser which has been bridged across the main circuit. Another central energy system is that of Stone, where impedance coils, or wire windings of considerable self-induction, are placed in the circuit instead of the repeating coils of the Hayes system. The effect of these coils is to prevent the rapidly alternating current in the telephone circuit flowing through the battery and to have it travel along the line to the other station where the impulses are reproduced. In the Dean-Carty System an impedance coil is employed bridged across the main circuit at the plugs and to the centre of it is connected one pole of a battery whose other terminal is grounded. There is also an impedance coil at each station connected with both sides of the circuit and its centre point is connected with the transmitter and the primary of the induction coil which are connected with the ground. All of the various systems for central energy in actual practice are necessarily exceedingly complex and are subject to important modification and improvement, but the foregoing are the more important. There are also methods where storage cells are used at the subscriber's station and where a thermopile is employed in connection with an ordinary lighting circuit.
Long-distance telephony was first made possible in 1885, when the American Bell Telephone Company organized the American Telephone and Telegraph Company. For several years previously experimental lines with metallic circuits of hard drawn copper were operated between New York and Boston. In 1885 a regular line between New York and Philadelphia was constructed, and so great was its success that within two years long-distance lines were established between New York and Boston, Albany and Buffalo, Chicago and Milwaukee, Boston and Providence, and New York and New Haven. The New York and Chicago circuit, 950 miles distance, 1900 miles of wire, was opened October 18, 1892, and long-distance telephoning between New York and Milwaukee and Saint Louis is also carried on. The American Bell Telephone Company in 1900 acquired the rights of patents of M. I. Pupin, by which the limits of long-distance telephony are greatly increased and conversation over circuits where there are submarine conductors of considerable length is possible. This is accomplished
