Wonderful as the transmission of signals over great distances is, the transmission of human speech so clearly that individual voices may be distinguished hundreds of miles away is even more so. Yet the instrument which works the miracle is essentially simple in its principles.
THE BELL TELEPHONE.
[Ill.u.s.tration: FIG. 62.--Section of a Bell telephone.]
The first telephone that came into general use was that of Bell, shown in Fig. 62. In a central hole of an ebonite casing is fixed a permanent magnet, M. The casing expands at one end to accommodate a coil of insulated wire wound about one extremity of a magnet. The coil ends are attached to wires pa.s.sing through small channels to terminals at the rear. A circular diaphragm, D, of very thin iron plate, clamped between the concave mouthpiece and the casing, almost touches the end of the magnet.
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, pa.s.sing through the central hole in the cover, make the diaphragm vibrate towards and away from the magnet. 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) emanating from the magnet, and these, cutting through the turns of the coil, induce a current in the line circuit. As the diaphragm approaches the magnet 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 pa.s.sing through its coil increase or diminish the magnetism of the magnet, and cause it to attract its diaphragm with varying force. The vibration of the 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 transmitter and a receiver, and the permanent magnetism of the cores renders it independent of an electric battery. But currents generated by it are so minute that they cannot overcome the resistance of a long circuit; therefore a battery is now always used, and with it a special device as transmitter.
If in a circuit containing 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 pa.s.s through the circuit.
Edison introduced the first successful _microphone_ transmitter, in which a small platinum disc connected to the diaphragm pressed with varying force against a disc of carbon, each disc forming part of the circuit. Vibrations of the diaphragm caused current to flow in a series of rapid pulsations.
[Ill.u.s.tration: FIG. 63.--Section of a granular carbon transmitter.]
THE GRANULAR CARBON TRANSMITTER.
In Fig. 63 we have a section of a microphone transmitter now very widely used. It was invented, in its original form, by an English clergyman named Hunnings. Resting 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 s.p.a.ce between C and a carbon diaphragm, D, is packed with carbon granules, G G. C has direct contact with line terminal T, which screws into it; D with T^1 through the bra.s.s casing, screw S, and a small plate at the back of the transmitter. Voice vibrations compress G G, and allow current to pa.s.s more freely from D to C. This form of microphone is very delicate, and unequalled for long-distance transmission.
[Ill.u.s.tration: FIG. 64.--A diagrammatic representation of a telephonic circuit.]
GENERAL ARRANGEMENT OF A TELEPHONE CIRCUIT.
In many forms of subscriber"s instruments both receiver and transmitter are mounted on a single handle in such a way as to be conveniently placed for ear and mouth. For the sake of clearness the diagrammatic sketch of a complete installation (Fig. 64) shows them separated. The transmitters, it will be noticed, are located in battery circuits, including the primary windings 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 a.s.sume that the transmitters are, in the first instance, both hung on the hooks of the metallic switches, which their weight depresses to the position indicated by the dotted lines. The handle of the magneto-generator at the left-end station is turned, and current pa.s.ses 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 from the switch hooks. The switches rise against 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 transmitter T in primary coil P are magnified by secondary coil S for transmission through the line circuit, and affect both receivers. The same thing 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.
[Ill.u.s.tration: A TELEPHONE EXCHANGE.]
DOUBLE-LINE CIRCUITS.
The currents used in telephones pulsate very rapidly, but are very feeble. Electric disturbances caused by the proximity of telegraph 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 (two wires) is practically free from interference, though where a number of wires are hung on the same poles, speech-sounds may be faintly induced in one circuit from another. This defect is, however, minimized by crossing the wires about among themselves, so that any one line does not pa.s.s 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 number of switchboards, each containing 80 to 100 terminals, and attended to by an operator, usually a girl.
When a subscriber wishes to be connected to another subscriber, he either turns the handle of a magneto generator, which causes a shutter to fall and expose his number at the exchange, or simply depresses a key which works a relay at the exchange and lights a tiny electric lamp. The operator, seeing the signal, connects her telephone with the subscriber"s circuit and asks the number wanted. This given, she rings up the other subscriber, and connects the two circuits by means of an insulated wire cord having a spike at each end to fit the "jack" sockets of the switchboard terminals. The two subscribers are now in communication.
[Ill.u.s.tration: FIG. 65.--The headdress of an operator at a telephone exchange. The receiver is fastened over one ear, and the transmitter to the chest.]
If a number on switchboard A calls for a number on switchboard C, the operator at A connects her subscriber by a jack cord to a trunk line running to C, where the operator similarly connects the trunk line with the number asked for, after ringing up the subscriber. The central exchange of one town is connected with that of another by one or more trunk lines, so that a subscriber may speak through an indefinite number of exchanges. So perfect is the modern telephone that the writer remembers on one occasion hearing the door-bell ring in a house more than a hundred miles away, with which he was at the moment in telephonic connection, though three exchanges were in the circuit.
SUBMARINE TELEPHONY.
Though telegraphic messages are transmitted easily through thousands of miles of cable,[16] submarine telephony is at present restricted to comparatively short distances. When a current pa.s.ses through a cable, electricity of opposite polarity induced on the outside of the cable damps the vibration in the conductor. In the Atlantic cable, strong currents of electricity are poured periodically into one end, and though much enfeebled when they reach the other they are sufficiently strong to work a very delicate "mirror galvanometer" (invented by Lord Kelvin), which moves a reflected ray up and down a screen, the direction of the movements indicating a dot or a dash. Reversible currents are used in transmarine telegraphy. The galvanometer is affected like the coils and small magnet in Wheatstone"s needle instrument (p. 128).
Telephonic currents are too feeble to penetrate many miles of cable.
There is telephonic communication between England and France, and England and Ireland. But transatlantic telephony is still a thing of the future. It is hoped, however, that by inserting induction coils at intervals along the cables the currents may be "stepped up" from point to point, and so get across. Turning to Fig. 64, we may suppose S to be on sh.o.r.e at the English end, and S_2 to be the _primary_ winding of an induction coil a hundred miles away in the sea, which magnifies the enfeebled vibrations for a journey to S_3, where they are again revived; and so on, till the New World is reached. The difficulty is to devise induction coils of great power though of small size. Yet science advances nowadays so fast that we may live to hear words spoken at the Antipodes.
[16] In 1896 the late Li Hung Chang sent a cablegram from China to England (12,608 miles), and received a reply, in _seven minutes_.
Chapter IX.
DYNAMOS AND ELECTRIC MOTORS.
A simple dynamo--Continuous-current dynamos--Multipolar dynamos--Exciting the field magnets--Alternating current dynamos--The transmission of power--The electric motor--Electric lighting--The incandescent lamp--Arc lamps--"Series" and "parallel"
arrangement of lamps--Current for electric lamps--Electroplating.
In previous chapters we have incidentally referred to the conversion of mechanical work into electrical energy. In this we shall examine how it is done--how the silently spinning dynamo develops power, and why the motor spins when current is pa.s.sed through it.
We must begin by returning to our first electrical diagram (Fig. 50), and calling to mind the invisible "lines of force" which permeate the ether in the immediate neighbourhood of a magnet"s poles, called the _magnetic field_ of the magnet.
Many years ago (1831) the great Michael Faraday discovered that if a loop of wire were moved up and down between the poles of an electro-magnet (Fig. 66) a current was induced in the loop, its direction depending upon that in which the loop was moved. The energy required to cut the lines of force pa.s.sed in some mysterious way into the wire. Why this is so we cannot say, but, taking advantage of the fact, electricians have gradually developed the enormous machines which now send vehicles spinning over metal tracks, light our streets and houses, and supply energy to innumerable factories.
[Ill.u.s.tration: FIG. 66.]
The strength of the current induced in a circuit cutting the lines of force of a magnet is called its pressure, voltage, or electro-motive force (expressed shortly E.M.F.). It may be compared with the pounds-to-the-square-inch of steam. In order to produce an E.M.F. of one volt it is calculated that 100,000,000 lines of force must be cut every second.
The voltage depends on three things:--(1.) The _strength_ of the magnet: the stronger it is, the greater the number of lines of force coming from it. (2.) The _length_ of the conductor cutting the lines of force: the longer it is, the more lines it will cut. (3.) The _speed_ at which the conductor moves: the faster it travels, the more lines it will cut in a given time. It follows that a powerful dynamo, or mechanical producer of current, must have strong magnets and a long conductor; and the latter must be moved at a high speed across the lines of force.
A SIMPLE DYNAMO.
In Fig. 67 we have the simplest possible form of dynamo--a single turn of wire, _w x y z_, mounted on a spindle, and having one end attached to an insulated ring C, the other to an insulated ring C^1. Two small brushes, B B^1, of wire gauze or carbon, rubbing continuously against these collecting rings, connect them with a wire which completes the circuit. The armature, as the revolving coil is called, is mounted between the poles of a magnet, where the lines of force are thickest.
These lines are _supposed_ to stream from the N. to the S. pole.
In Fig. 67 the armature has reached a position in which _y z_ and _w x_ are cutting no, or very few, lines of force, as they move practically parallel to the lines. This is called the _zero_ position.
[Ill.u.s.tration: FIG. 67.]