they rested. The five on the right side of the stage were connected to one telephone cable, and the five on the left to another. These lines ran to the two salons d'audition in the Exhibition building, such as is pictured in the frontispiece. In each of these there were twenty pairs of telephones, one pair to each listener. The right hand telephone of a pair was connected to one of the two telephone cables, and the left hand one to the other, so that the music from both sides of the stage was brought to the ears of the listener; and thus the strains of the whole orchestra, or the voice of a singer moving across the stage from one side to the other, could be plainly heard. In fact, the rendering of the complex music from voice and band was remarkably correct and clear. One seemed to be listening to it from a box within the Opera House itself. There are other patterns of the Bell receiver, such as the "pony crown," the Hickley, the D'Arsonval; but those we have described are most often used. There are receivers, too, which differ altogether in principle from that of Bell, and we shall briefly describe the most promising of these.

The telephone of Professor A. E. Dolbear, acts by the attraction of one electrified metal plate on another, and not by electro-magnetism. It consists of two thin metal discs c D, Fig. 41, about two inches in diameter, placed very close together, but not touching. They are kept apart by a thin ring or washer of hard rubber; and enclosed between two caps of hard wood or ebonite. The cap in front of the plate c has a hole turned in it to serve as an earpiece to listen at. The front plate is free to vibrate at its middle, but an adjusting screw E bears upon the back plate, and adjusts it to the best

distance from the front plate. It may therefore be considered a fixed plate, not free to vibrate. These plates are connected to the line and "earth" by terminal wires, and the vocal currents charging that con

nected to the line attract that connected to the earth with a force proportional to their intensity. The front or free plate responds to this attraction by vibrating at its middle, and thus reproducing the sounds which originally provoked the vocal currents in the line.

These currents are produced by a microphone transmitter after the manner of Fig. 42, where T is the transmitter in circuit with a battery в and the primary circuit of an induction coil 1, the secondary circuit of

the coil being in circuit with the line, to which the telephones R R are connected. No wires are shown connecting these instruments to "earth" because, though convenient in practice, this is not absolutely necessary, as the listener himself forms a good enough earth connection under ordinary circumstances, provided he

grasp the telephone so as to bring his hand into contact with the adjusting screw E, Fig. 41, and thus connects the back plate D to his person.

Dolbear's telephone is in reality an air "condenser: and the ordinary telegraph condenser has also been employed to receive music by Mr. C. F. Varley and speech by M. A. Dunand. Such a condenser is made by taking sheets of tinfoil and laying them one over another with sheets of paraffined paper in between. The foils 1, 3, 5, etc. are connected together to form one pole a of the condenser, and the foils 2, 4, 6, etc. to form the other pole. These poles correspond to the terminals

G

and the separate sets of foil to the metal plates of Dolbear's telephone. They can be used in the same way as a telephone receiver: but in order to get a good effect M. Dunand keeps them charged with a battery, and the vocal currents merely cause fluctuations in this permanent charge. Any condenser will answer, but in M. Dunand's there are thirty sheets of tinfoil, each 6 centimetres square, separated by sheets of dry paper, and joined alternately to form the poles; the sheets are divided into ten sets of three each with air-spaces between, but all are connected together. Two small condensers enclosed in boxes with holes for hearing at are sometimes mounted on the ends of a bent spring which fits the top of the head and presses the receivers to the ears.

Other telephone receivers have been based on the fact that an iron rod expands and contracts when surrounded by a coil of wire through which the vocal currents pass. Such is the telephone without a tympan of M. Ader, and that with an ebonite tympan of Mr. Barnay. Mr. W. H. Preece, too, has obtained speech from a stretched platinum wire fixed at one end and attached at the other to the centre of a vibrating diaphragm. On passing the vocal currents from the line through this wire its sympathetic elongation and contraction caused the diaphragm to vibrate and emit the corresponding words. These and other devices are, however, not in practical use, so that we need not dwell upon them here.

Telephones are either used on private lines between two correspondents, or on public lines between a great number of correspondents by the "exchange" system. The telephone exchange is a web of telephone circuits

all radiating from one central station to separate houses in a district or town, so that any person renting the use of a circuit can speak to any other client of the exchange by asking the attendant at the central station to connect his circuit to that of the client in question. Thus, if Brown, of Mark Lane, E.C., wishes to speak with his friend Robinson, of Cheapside, he simply goes to the telephone hanging up in his office and calling up the attendant at the central station of the United Telephone Company in Coleman Street, requests to be put in communication with Robinson. The clerk replies, "All right;" then, by means of a "switch" board, to which all the ends of the circuits radiating from the station are brought, he quickly joins the circuits of Brown and Robinson, and leaves them to talk together until they signify that they have done. In this way much time is saved in keeping personal appointments; and business is greatly facilitated. One curious result of the exchange system is that outlying property has risen in value, owing to the annihilation of distance in business transactions.

Several ingenious apparatus have been invented for promptly working the exchanges; but as they are mere accessories to the telephone we shall not describe them here. We ought to mention, however, the automatic exchange of Mr. Connolly, and that of Messrs. Brown and Saunders, by which a small group of subscribers can communicate with each other without the assistance or intervention of a central office. This improvement not only saves time but is more confidential in its operation than the public exchange, and will no doubt supply a special want.