in this way were too feeble to produce satisfactory sounds at the receiving instrument when there was a line of any considerable length, and it was necessary to find a better transmitter. Accordingly there followed the carbon transmitter, where the change of resistance of carbon under pressure produces pulsations in the current corresponding to the vibration of the sound waves.
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 current passed through the needle and the liquid and the resistance of the circuit varied with the vibration of the diaphragm. This transmitter allowed variable but continuous currents to pass, and did not interrupt them entirely, as did the transmitter of Reis's telephone. The next transmitter 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 (1878), 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 comparison 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 receiver has been removed from the hook (hook up), shows the general arrangement of the circuit. 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 transmitter will give rise to induced currents in the secondary, which will affect the receiver correspondingly, and the original vibration of the sound waves will be reproduced. The addition of the coil renders the apparatus far more sensitive 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 connection with a metal diaphragm. The transmitter in most general use at present is the White or solid back transmitter, illustrated 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 contact 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 the rest of the apparatus and is connected with the other conductor. There are numerous other forms of transmitter which are constantly increasing with the extending use of the telephone for interior use and by independent companies. These transmitters 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.
WHITE SOLID BACK TRANSMITTER.
With the telephone must be 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 revolved by a few turns of a crank and a current 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 alternating and the bell used is of the polarized ringing form, where the armature is alternately attracted by one pole and the other, depending 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 machine and bell are in circuit as long as the receiver 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 car 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 station 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 appropriate number or an incandescent lamp, informs 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
