THE TELEPHONE.
The Bell telephone--The Edison transmitter--The granular carbon
transmitter--General arrangement of a telephone
circuit--Double-line circuits--Telephone exchanges--Submarine
telephony.
For the purposes of everyday life the telephone is even more useful than
the telegraph. Telephones now connect one room of a building with
another, house with house, town with town, country with country. An
infinitely greater number of words pass over the telephonic circuits of
the world in a year than are transmitted by telegraph operators. The
telephone has become an important adjunct to the transaction of business
of all sorts. Its wires penetrate everywhere. Without moving from his
desk, the London citizen may hold easy converse with a Parisian, a New
Yorker with a dweller in Chicago.
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.
[Illustration: 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 passing 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, passing 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 passing 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 pass 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.
[Illustration: 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 space 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 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 form of microphone is very delicate, and unequalled for
long-distance transmission.
[Illustration: 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 assume 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 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 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.
[Illustration: 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
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 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.
[Illustration: 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 passes 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 shore 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_.