Part 9 (1/2)

Wonderful as the transreat distances is, the transmission of huuished hundreds of miles away is even more so Yet the instrument which works the miracle is essentially simple in its principles

THE BELL TELEPHONE

[Illustration: FIG 62--Section of a Bell telephone]

The first telephone that ca 62 In a central hole of an ebonite casing is fixed a per expands at one end to accommodate a coil of insulated ound about one extrenet The coil ends are attached to wires passing through sm, D, of very thin iron plate, cla, alnet

We will suppose that two Bell telephones, A and B, are connected up by wires, so that the wires and the coils form a complete circuit Words are spoken into A The air vibrations, passing through the central hole in the cover, net The distances through which the diaphragm moves have been measured, and found not to exceed in some cases more than 1/10,000,000 of an inch! Its movements distort the shape of the ”lines of force” (see p 118) eh the turns of the coil, induce a current in the line circuit As the diaphragnet a circuit is sent in one direction; as it leaves it, in the other Consequently speech produces rapidly alternating currents in the circuit, their duration and intensity depending on the nature of the sound

Now consider telephone B The currents passing through its coil increase or dinet, and cause it to attract its diaphragm disturbs the air in exact accordance with the vibrations of A's diaphragm, and speech is reproduced

THE EDISON TRANSMITTER

The Bell telephone may be used both as a transnetism of the cores renders it independent of an electric battery But currents generated by it are socircuit; therefore a battery is noays used, and with it a special device as trans a telephone and a battery there be a loose contact, and this be shaken, the varying resistance of the contact will cause electrical currents of varying force to pass through the circuit

Edison introduced the first successful _microphone_ transmitter, in which a sainst a disc of carbon, each disc form caused current to flow in a series of rapid pulsations

[Illustration: FIG 63--Section of a granular carbon trans 63 we have a section of a microphone transinal for in a central cavity of an ebonite seating is a carbon block, C, with a face moulded into a number of pyramidal projections, P P The space between C and a carbon diaphragranules, G G C has direct contact with line terh the brass casing, screw S, and a small plate at the back of the transmitter Voice vibrations compress G G, and allow current to pass more freely from D to C This for-distance transrammatic representation of a telephonic circuit]

GENERAL ARRANGEMENT OF A TELEPHONE CIRCUIT

In many forms of subscriber's instrule handle in such a way as to be conveniently placed for ear and ra 64) shows them separated The transmitters, it will be noticed, are located in battery circuits, including the pris P P_2 of induction coils The transmitters are in the line circuit, which includes the secondary windings S S_2 of the coils

We will assu on the hooks of the ht depresses to the position indicated by the dotted lines The handle of the enerator at the left-end station is turned, and current passes through the closed circuit:--Line A, E B_2, contact 10, the switch 9; line B, 4, the other switch, contact 5, and E B Both bells ring Both parties now lift their receivers froainst contacts 1, 2, 3 and 6, 7, 8 respectively Both primary and both secondary circuits are now completed, while the bells are disconnected from the line wires The pulsations set up by transnified by secondary coil S for transh the line circuit, and affect both receivers The sa happens when T_2 is used At the end of the conversation the receivers are hung on their hooks again, and the bell circuit is remade, ready for the next call

[Illustration: A TELEPHONE EXCHANGE]

DOUBLE-LINE CIRCUITS

The currents used in telephones pulsate very rapidly, but are very feeble Electric disturbances caused by the proxiraph or tram wires would much interfere with them if the earth were used for the return circuit It has been found that a complete metallic circuit (tires) is practically free fro on the same poles, speech-sounds may be faintly induced in one circuit fro the wires about a themselves, so that any one line does not pass round the corresponding insulator on every pole

TELEPHONE EXCHANGES

In a district where a number of telephones are used the subscribers are put into connection with one another through an ”exchange,” to which all the wires lead One wire of each subscriber runs to a common ”earth;”

the other terminates at a switchboard presided over by an operator In an exchange used by many subscribers the terminals are distributed over a nu 80 to 100 terirl