Telephone

Telephone, is an instrument for transmitting sounds to a distance. Sounds directed upon a diaphragm or stretched membrane cause the latter to vibrate, and if a second diaphragm is connected with the first by means of a tight string or wire attached to the centre of each, the vibrations of one will be communicated to the other. Although speech may be transmitted In this way to a limited distance, the difficulty of guiding the string round corners without interfering with its vibrations rendes the arrangement of small importance. It appears to have first occurred to Philip Reis to endeavour to make the vibrations of the one diaphragm produce corresponding changes in an electric current, and to make this varying current produce sounds at the distant end of the circuit. He attached a piece of metal to the centre of a diaphragm of bladder, which made and broke contact as the membrane vibrated. It had previously been noted by Page that an electro-magnet produces a sound as its circuit is made or broken, and consequently Reis was enabled, by simply leading the intermittent urrent from his diaphragm to an electromagnet to reproduce sounds to some extent. but this device did little more than give a sound having the same pitch as the original, whereas, in order to transmit speech, the form of the vibrations must also be retained. In 1874 Bell began to make experiments in the same direction, and ultimately succeeded in devising a practicable telephone. In this instrument the sound impinges upon a diaphragm of thin sheet-iron supported round its edge; behind the diaphragm, and opposite its centre, is a short piece of iron surrounded by a coil of fine wire. This iron core forms the pole piece of a steel magnet, and, as the diaphragm in vibration approaches or recedes from it, the reslstance of the magnetic circuit is varied and the number of lines of force which pass through the coil suffer a corresponding change. If now the terminals of this coil are connected to the coil of an exactly similar instrument, a current will be generated in the circuit thus formed which will vary in direction and strength in proportion to the motions of the iron diaphragm. But as this current also circulates in the coil of the second instrument the strength of its magnet will be varied, and its iron diaphragm will be attracted more or less strongly, in accordance with the variation of the current and the motion of the diaphragm of the first instrument. It will thus vibrate in a way which is an almost exact reproduction of that of the transmitting instrument, and any sound will be reproduced with considerable accuracy though with some loss in loudness. In order to overcome this last drawback Reis's transmitter was revived in a modified form by Edison, Hughes, Berliner, and many others. In all these carbon transmitters or microphones the vibrations of a diaphragm are caused to vary the resistance of the circuit, instend of making and breaking it as Reis's did. Two conductors - either both of carbon, or one of carbon and one of platinum - are placed loosely in contact, and it is found that vibrations communicated to such a contact cause its resistance to vary. By placing a battery in the circuit a current which varies with the sound-vibrations is obtained, and such an arrangement used in conjunction with Bell's instrument as a receiver constitutes the modern telephone, The resistance of a carbon tramsmitter is not very high and if placed in series with a long line of wire would form but a small portion of the resistance of the whole circult; hence even a large variation in the resistance of the contact would produce a small change in the total resistance, and therefore in the current. It is consequently found better to connect the microphone and battery, not to the line, but to the primary winding of a small induction coil (the resistance of which is low), and to connect the line to the secondary winding. In this way a fairly strong current in the primary circuit is converted into a current of much higher potential, suited to overcome the line resistance. In a telephone exchange system wires from each subscriber's instrument are taken to a central office or exchange, and arrangements are made for connecting any subscriber with any other. This system is further extended by connecting the exchanges in various towns by trunk lines. In the first telephone systems erected, single lines, with earth returns, were used, and from this many troubles have arisen. If a telegraph wire runs parallel and near to a telephone wire, the currents in the former induce currents in the latter, and various return currents are apt to get mixed; further, messages may get from one wire to another by induction or leakage, and the efficiency of the telephone service is very much reduced. The remedy for these evils is to use a second wire as a return, and in places liable to induction, to cross the wires at certain points, so that the average distance of the two wires from the source of disturbance is the same.