Previous to this, however, came Elisha Gray's transmitter, consisting of a needle mounted at the centre of a vibrating diaphragm and dipping into a liquid of rather low conductivity. The cur rent passed through the needle and the liquid and the resistance of the circuit varied with the vi bration of the diaphragm. This transmitter al lowed variable but continuous currents to pass, and did not interrupt them entirely, as did the transmitter of Reis's telephone. The next trans mitter was that of Berliner (1877), based on the variation of resistance with pressure, a diaphragm vibrating in contact with a metal knob; and then came that of Edison, where a button of compressed carbon was in contact with a small disk of platinum on the diaphragm. The microphone of Hughes (187S), where two bodies are in loose contact, has furnished the type of modern transmitters, where the carbon is in the form of finely divided granules held between two conducting plates, one of which is the diaphragm on which the voice strikes. The Edison form was, however, used for a number of years, but the Hunnings form or some modification was found necessary when long-distance telephoning came into practice. With such transmitters it was found necessary to introduce a secondary circuit instead of having the varying currents flow in the main circuit as was the case originally. The change in resistance then was small in compari son with the total resistance of the circuit and the effect on the receiver was not as marked as desired. Accordingly Edison conceived the idea of using an induction coil in the circuit with the transmitter. This induction coil consists of a primary coil with a few layers of coarse copper wire wound around a bundle of soft iron wire and a secondary coil of a large number of turns of fine wire. The diagram below, where the re ceiver has been removed from the hook (hook up), shows the general arrangement of the cir cuit. The current from the battery flows through the transmitter and the primary of the induction coil. Any difference of intensity in current caused by a change of resistance in the trans mitter will give rise to induced currents in the secondary, which will affect the receiver corre spondingly, and the original vibration of. the sound waves will be reproduced. The addition of the coil renders the apparatus far more sensi tive and the increase in voltage caused by the many turns of the secondary enables the sound to be transmitted to a much greater distance. The Blake transmitter, the invention of Francis Blake, of Boston, was used in the United States almost universally until the adoption of the transmitter with the granulated carbon, and is familiar in the older instruments, being incased in a wooden box. Its action depends upon the pressure on a button of a compressed carbon by a point in con nection with a metal diaphragm. The trans mitter in most general use at present is the White or solid back transmitter, ii lustrated in the accompanying diagram. The construction of the transmitter will appear from the diagram. Between two disks of carbon is placed granular carbon. One disk is in con tact with the solid back of the instrument, while the other is in contact with the diaphragm on which the sound waves impinge. The back of the transmitter forms one electrode, while the front plate is insulated from time rest of the ap paratus and is connected with the other conduc tor. There are numerous other forms of trans mitter which are constantly increasing with the extending use of the telephone for interior use and by independent companies. These trans mitters require batteries, of which the Leclanche, some form of dry cell, or the Fuller bichromate of potash cell are generally employed, while for central stations and wherever possible storage cells are very desirable and useful.
With the telephone must he included some sort of call-bell, and for this purpose a magneto-bell is usually employed. This consists of a small magneto-generator in which the armature is re volved by a few turns of a crank and a cur rent of considerable voltage (that is, compared with the battery) is transmitted to the distant station, where there is what is known as a polarized bell. The current transmitted is al ternating and the bell used is of the polar ized ringing form, where the armature is alter nately attracted by one pole and the other, de pending on the polarity of the passing current. In the older form of circuit still in use in many exchanges where the central station must be called up by turning the crank the magneto ma chine and bell are in circuit as long as the re ceiver hangs on a hook which forms a switch. The circuit is shown below. When the handle of the magneto is turned a current is sent out through the bell and hook to the line. Then the receiver is taken down and placed at the ear and the bell circuit is broken, merely the telephone circuit being in connection with the line wire. With a number of subscribers an exchange or central sta tion is necessary where the wires connecting the various subscribers or other stations can be joined at will. At these stations an annunciator, which may be either a falling flap with the ap propriate number or an incandescent lamp, in forms the operator of the call and by means of the switchboard to which all the wires are led the connections desired by the subscriber are made. Improvements in switchboards and their
increased size to meet the growing demands- of cities have been most marked, and this form of apparatus is now so complex as to be quite un intelligible except to a telephone engineer, as almost every part of the apparatus has been sub jected to important improvements.
The use of a common battery at the central station has been one of the most important de velopments 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 in forming 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 con sists 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 cir cuit wires. In other words, the negative pole of the battery connects with the two sets of wind ings 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 1111 the transmitter of one circuit will produce similar efl•ects in the other circuit by inductive action. is case when the line is arranged for talking. To signal the central sta tion the subscriber removes his re ceiver from the hook, thus closing a circuit which acts upon a relay mid causes a small incandescent lamp to glow. This gives the signal to the central operator, who immediately eompletes the talking circuit first with her own trans mitter and revolver, and then with that of the desired subscriber. The attention of the latter has been at tracted by a call hell energized by an alternating current. This hell is in series with a condenser which has been bridged across the main circuit. Another central energy system is that of Stone, where im pedance 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 pre vent the rapidly alternating current in the tele phone 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 cen tral energy in actual practice are necessarily exceedingly complex and are subject to im portant 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 pos sible in 1885, when the American Bell Telephone Company organized the American Telephone and .Telegraph Company. For several years previous ly 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 Chi cago circuit, 950 miles distance, 1900 miles of wire, was opened October 18. 1892. and long-dis tance telephoning between New York and :NM waukee and Saint Louis is also carried on. The American Bell Telephone Company in 1900 ac quired the rights of patents of N. I. Pupin, by which the limits of long-distance telephony are greatly increased and conversation over circuits where there are submarine conductors of consid erable length is possible. This is accomplished by inserting coils of self-induction at regularly recurring intervals along the line obtained by calculation.
The American Bell Telephone Company prac tically controls the telephone business of the United States, though a number of strong inde pendent companies have been organized, whose business is increasing.
success of the plant, but apparently the advice of the engineer was not followed during construc tion, while subsequently the plant suffered from poor management. It is said that the municipal plant resulted in a marked reduction of rates and that when sold it yielded a profit. It should be added that in 1911 all telephone licenses granted to private companies in Great Britain will ex