THE TELEPHONE
In our younger days most of us have amused ourselves with a toy
telephone consisting of a long piece of string having each end passed
through the bottom of a little cardboard box, and secured by a knot.
If the string is stretched tightly this arrangement enables whispered
words to be heard at a distance of 20 or 30 yards. Simple as is this
little toy, yet it is probable that many people would be rather
nonplussed if asked suddenly to explain how the sounds travel along
the string from one box to the other. If the toy had some complicated
mechanism most likely every one would want to know how it worked, but
the whole thing is so extremely simple that generally it is dismissed
without a thought.
If we strike a tuning-fork and then hold it close to the ear, we hear
that it produces a sound, and at the same time, from a slight sensation
in the hand, we become aware that the fork is in vibration. As the fork
vibrates it disturbs the tiny particles of air round it and sets them
vibrating, and these vibrations are communicated from one particle to
another until they reach the drum of the ear, when that also begins to
vibrate and we hear a sound. This is only another way of saying that
the disturbances of the air caused by the vibrations of the tuning-fork
are propagated in a series of waves, which we call “sound waves.” Sound
is transmitted better through liquids than through the air, and better
still through solids, and this is why words spoken so softly as to be
inaudible through the air at a distance of, say, 100 feet, can be heard
fairly distinctly at that distance by means of the string telephone.
The sound reaches us along the string in exactly the same way as
through the air, that is, by means of minute impulses passed on from
particle to particle.
A more satisfactory arrangement than the string telephone consists of
two thin plates of metal connected by a wire which is stretched very
tightly. Words spoken close to one plate are heard by a listener at the
other plate up to a considerable distance. Let us try to see exactly
what takes place when this apparatus is used. In the act of speaking,
vibrations are set up in the air, and these in turn set up vibrations
in the metal plate. The vibrations are then communicated to the wire
and to the metal plate at the other end, and finally the vibrations
of this plate produce vibrations in the air between the plate and the
listener, and the sound reaches the ear.
This simple experiment shows the remarkable fact that a plate of
metal is able to reproduce faithfully all the vibrations communicated
to it by the human voice, and from this fact it follows that if we
can communicate the vibrations set up in one plate by the voice, to
another plate at a distance of 100 miles, we shall be able to speak
to a listener at the further plate just as if he were close to us. A
stretched string or wire transmits the vibrations fairly well up to
a certain distance, but beyond this distance the vibrations become
weaker and weaker until no sound at all reaches the air. By the aid of
electricity, however, we can transmit the vibrations to a tremendous
distance, the range being limited only by the imperfections of our
apparatus.
The first attempt at the construction of an electric telephone, that
is an instrument by means of which the vibrations set up by the voice
or by a musical instrument are transmitted by electricity, was made in
1860 by Johann Philipp Reis, a teacher in a school at Friedrichsdorf,
in Germany. His transmitting apparatus consisted of a box having a
hole covered by a tightly stretched membrane, to which was attached
a little strip of platinum. When the membrane was made to vibrate by
sounds produced close to the box, the strip of platinum moved to and
fro against a metal tip, which closed the circuit of a battery. The
receiver was a long needle of soft iron round which was wound a coil
of wire, and the ends of the needle rested on two little bridges of
a sounding box. The vibrations of the membrane opened and closed the
circuit at a great speed, and the rapid magnetization of the needle
produced a tone of the same pitch as the one which set the membrane
vibrating. This apparatus transmitted musical sounds and melodies with
great accuracy, but there is considerable difference of opinion as to
whether it was able to transmit speech. Professor Sylvanus Thompson
distinctly states that Reis’s telephone could and did transmit speech,
but other experts dispute the fact. We probably shall be quite safe
in concluding that this telephone did transmit speech, but very
imperfectly. In any case it is certain that the receiver of this
apparatus is not based on the same principle as the modern telephone
receiver.
Some years later Graham Bell, Professor of Vocal Physiology in the
University of Boston, turned his attention to the electric transmission
of speech, probably being led to do so from his experiments in
teaching the deaf and dumb. His apparatuses shown at an exhibition
in Philadelphia in 1876, consisted of a tube having one end open for
speaking into, and the other closed by a tightly stretched membrane to
which was attached a very light steel bar magnet. The vibrations set
up in the membrane by the voice made the little magnet move to and
fro in front of the poles of an electro-magnet, inserted in a battery
circuit, thus inducing currents of electricity in the coils of the
latter magnet. The currents produced in this way varied in direction
and strength according to the vibratory movements of the membrane,
and being transmitted along a wire they produced similar variations
in current in another electro-magnet in the receiver. The currents
produced in this manner in the receiver set up vibrations in a metal
diaphragm in front of the magnet poles, and so the words spoken into
the transmitter were reproduced.
Since the year 1876 the telephone has developed with remarkable
rapidity, and an attempt to trace its growth would involve a series
of detailed descriptions of closely similar inventions which would
be quite uninteresting to most readers. Now, therefore, that we have
introduced the instruments, and seen something of its principle and
its early forms, it will be most satisfactory to omit the intermediate
stages and to go on to the telephone as used in recent years. The first
telephone to come into general use was the invention of Graham Bell,
and was an improved form of his early instrument just described. A case
or tube of ebonite, which forms the handle of the instrument, contains
a steel bar magnet having a small coil of insulated wire at the end
nearest the mouthpiece of the tube, the ends of the coil passing along
the tube to be connected to the line wires. Close to the coil end of
the magnet, and between it and the mouthpiece, is fixed a diaphragm of
thin sheet-iron. A complete outfit consists of two of these instruments
connected by wires, and it will be noticed that no battery is employed.
The air vibrations set up by the voice make the diaphragm vibrate
also, so that it moves backwards and forwards. These movements are
infinitesimally small, but they are sufficient to affect the lines of
force of the magnet to such an extent that rapidly alternating currents
of varying degrees of strength are set up in the coil and sent along
the line wire. On arriving at the receiver these currents pass through
the coil and produce rapid variations in the strength of the magnet, so
that instead of exerting a uniform attraction upon the iron diaphragm,
the magnet pulls it with constantly varying force, and thus sets it
vibrating. The air in front of the diaphragm now begins to vibrate,
and the listener hears a reproduction of the words spoken into the
transmitter. The way in which the fluctuations of the current make the
second diaphragm vibrate exactly in accordance with the first is very
remarkable, and it is important to notice that the listener does not
hear the actual voice of the speaker, but a perfect reproduction of it;
in fact, the second diaphragm speaks.
The reader probably will be surprised to be told that the transmitter
and the receiver of a magneto-electric telephone are respectively a
dynamo and electric motor of minute proportions. We provide a dynamo
with mechanical motion and it gives us electric current, and by sending
this current through an electric motor we get mechanical motion
back again. In the transmitter of the telephone just described, the
mechanical motion is in the form of vibrations of the metal diaphragm,
which set up currents of electricity in the coil of wire round the
magnet, so that the transmitter is really a tiny dynamo driven by
the voice. The receiver is provided with electric current from the
transmitter, and it converts this into mechanical motion in the
diaphragm, so that the receiver is a little electric motor.
Transmitters of the type just described work well over short distances,
but the currents they produce are too feeble for transmission over
a very long wire, and on this account they have been superseded
by transmitters on the microphone principle. A microphone is an
instrument for making extremely small sounds plainly audible. If
a current is passed through a box containing loose bits of broken
carbon, it meets with great resistance, but if the bits of carbon are
compressed their conducting power is considerably increased. Even
such slight differences in pressure as are produced by vibrating the
box will affect the amount of current passing through the carbon. If
this current is led by wires to an ordinary telephone receiver the
arrangement becomes a simple form of microphone. The vibrations of the
box vary the resistance of the carbon, and the corresponding variations
in the current set up vibrations in the receiver, but in a magnified
form. The smallest sound vibrations alter the resistance of the carbon,
and as these vibrations are magnified in the receiver, the reproduced
sound is magnified also. The footsteps of a fly may be heard quite
distinctly by means of a good microphone, and the ticks of a watch
sound like the strokes of a hammer.
[Illustration: FIG. 31.--Diagram of Microphone Transmitter.]
By means of this power of magnifying vibrations a microphone
transmitter can be used on a line of tremendous length, where an
ordinary Bell transmitter would be utterly useless. The general
features of this transmitter, Fig. 31, are a diaphragm and a block
of carbon separated slightly from one another, the intervening space
being filled with granules of carbon. These are enclosed in a case of
ebonite having a mouthpiece in front and two terminals behind, one
terminal being connected with the carbon block and the other with the
diaphragm. From these terminals wires are led to a battery and to the
receiver, which is of the Bell type. The current has to pass through
the carbon granules, and the movements of the diaphragm when set in
vibration by the voice vary the pressure upon the granules, and in this
way set up variations in the current. Carbon dust also may be used
instead of granular carbon, and then the instrument is called a “dust
transmitter.”
[Illustration: FIG. 32.--Combined Telephone Transmitter and Receiver.]
It is usual to have a transmitter and a receiver on one handle for
the greater convenience of the user. The arrangement is shown in
Fig. 32, and it will be seen that when the user places the receiver
to his ear the transmitting mouthpiece is in position for speaking.
The microphone with its carbon dust is placed at A, just below the
mouthpiece, and the earpiece or receiver B contains a little magnet and
coil with a diaphragm in front, so that it is really a Bell instrument.
A little lever will be noticed at C. This is a switch which brings the
transmitter into circuit on being pressed with the finger.
It is now time to see something of the arrangement and working
of telephone systems. As soon as the telephone became a
commercially practicable instrument the necessity for some means of
inter-communication became evident, and the telephone exchange was
brought into being. The first exchange was started in 1877, in Boston,
but this was a very small affair and it was run on very crude lines.
When one subscriber wished to communicate with another he had to call
up an operator, who received the message and repeated it to the person
for whom it was intended; there was no direct communication between
the various subscribers’ instruments. As the number of users increased
it became necessary to devise some system whereby each subscriber could
call the attention of an operator at the central station, and be put
into direct communication with any other subscriber without delay; and
the exchange system of to-day, which fulfils these requirements almost
to perfection, is the result of gradual improvements in telephone
methods extending over some thirty-five years.
When a subscriber wishes to telephone, he first must call up the
operator at the exchange. Until comparatively recently this was done
by turning a handle placed at the side of the instrument. This handle
operated a little dynamo, and the current produced caused a shutter at
the exchange to drop and reveal a number, just as in the electric bell
indicator, so that the operator knew which instrument was calling. As
soon as the operator answered the call, the shutter replaced itself
automatically. The signal to disconnect was given in the same way, but
the indicator was of a different colour in order to prevent confusion
with a call signal. These handle-operated telephones are still in
common use, but they are being replaced by instruments which do away
with handle-turning on the part of the subscriber, and with dropping
shutters at the exchange. In this latest system all that the subscriber
has to do is to lift his telephone from its rest, when a little
electric lamp lights up at the exchange; and when he has finished his
conversation he merely replaces the telephone, and again a little lamp
glows.
We must now see what happens at the exchange when a call is made. Each
operator has control of a number of pairs of flexible cords terminating
in plugs, the two cords of each pair being electrically connected. The
plugs rest on a shelf in front of the operator, and the cords pass
through the shelf and hang down below it. If a plug is lifted, the cord
comes up through the shelf, and it is drawn back again by a weight
when the plug is not in use. Two lamps are provided for each pair of
cords, one being fixed close to each cord. The two wires leading from
each subscriber’s instrument are connected to a little tube-shaped
switch called a “jack,” and each jack has a lamp of its own. When a
subscriber lifts his telephone from its rest a lamp glows, and the
operator inserts one plug of a pair into the jack thus indicated, and
the lamp goes out automatically. She then switches on her telephone to
the caller and asks for the number of the subscriber to whom he wishes
to speak; and as soon as she gets this she inserts the other plug of
the pair into the jack belonging to this number. By a simple movement
she then rings up the required person by switching on the current to
his telephone bell.
Here comes in the use of the two lamps connected with the cords. As
long as the subscribers’ telephones are on their rests the lamps are
lighted, but as soon as they are lifted off the lamps go out. The
caller’s telephone is of course off its rest, and so the lamp connected
with the first cord is not lit; but until the subscriber rung up lifts
his instrument to answer the call, the lamp of the second cord remains
lit, having first lighted up when the plug was inserted in the jack
of his number. When the second lamp goes out the operator knows that
the call has been responded to, and that the two subscribers are in
communication with each other. Having finished their conversation,
both subscribers replace their instruments on the rests, whereupon
both lamps light up, informing the operator that she may disconnect by
pulling out the plugs.
It is manifestly impossible for one operator to attend to the calls of
all the subscribers in the exchange, and so a number of operators are
employed, each one having to attend to the calls of a certain number
of subscribers. At the same time it is clear that each operator may be
called upon to connect one of her subscribers to any other subscriber
in the whole exchange. In order to make this possible the switchboard
is divided into sections, each having as many jacks as there are lines
in the exchange, so that in this respect all the sections are multiples
of each other, and the whole arrangement is called a “multiple
switchboard,” the repeated jacks being called “multiple jacks.” Then
there are other jacks which it is not necessary to duplicate. We have
seen that when a subscriber calls the exchange a lamp glows, and
the operator inserts a plug into the jack beside the lamp, in order
to answer the call and ascertain what number is required. These are
called “answering jacks,” and the lamp is the line signal. It is usual
to have three operators to each section of the switchboard, and each
operator has charge of so many answering jacks, representing so many
subscribers. At the same time she has access to the whole section, so
that she can connect any of her subscribers to any other line in the
exchange.
When a number is called for, the operator must be able to tell at
once whether the line is free or not. The jack in her section may be
unoccupied, but she must know also whether all the multiple jacks
belonging to that number are free, for an operator at another section
may have connected the line to one of her subscribers. To enable an
operator to ascertain this quickly an electrical test is provided. When
two lines are connected, the whole of the multiple jacks belonging to
each are charged with electricity, and if an operator at any section
touches one of these jacks with a plug, a current through her receiver
makes a click, and on hearing the click she knows that the line is
engaged. The testing takes an extremely short time, and this is why a
caller receives the reply, “Number engaged,” so promptly that he feels
inclined to doubt whether the operator has made any attempt at all to
connect him up to the number.
In order that an operator may have both hands free to manipulate the
plugs, her telephone receiver is fixed over one ear by a fastening
passing over her head, and the transmitter is hung from her shoulders
so as to be close to her mouth.
In telegraphy it is the rule to employ the earth for the return part of
the circuit, but this is not customary in telephony. The telephone is
a much more sensitive instrument than the telegraph, and a telephone
having an earth return is subject to all kinds of strange and weird
noises which greatly interfere with conversation. These noises may
be caused by natural electrical disturbances, or by the proximity of
telegraph and other wires conveying electric currents. On this account
telephone lines are made with a complete metallic circuit. As in
telegraphy, protection from lightning flashes is afforded by lightning
arresters. The current for the working of a telephone exchange is
supplied from a central battery of accumulators, and also from dynamos.
[Illustration: PLATE XII.
_By permission of_ _Craven Brothers Ltd._
LARGE ELECTRIC TRAVELLING CRANE AT A RAILWAY WORKS.]
Although the manual exchange telephone system of to-day works with
remarkable efficiency, it has certain weak points. For instance, if
an operator cares to do so, she can listen to conversations between
subscribers, so that privacy cannot be assured. As a matter of
fact, the operators have little time for this kind of thing, at any
rate during the busy hours of the day, and as a rule they are not
sufficiently interested in other people’s affairs to make any attempt
to listen to their remarks. The male operators who work through the
slack hours of the night are occasionally guilty of listening. Some
time ago the writer had to ring up a friend in the very early
morning, and during the conversation this gentleman asked what time it
was. Before the writer had time to get a word out, a deep bass voice
from the exchange replied, “Half-past two.” Little incidents of this
sort remind one that it is not wise to speak too freely by telephone.
Then again operators are liable to make wrong connexions through faulty
hearing of the number called for, and these are equally annoying to the
caller and to the person rung up in mistake. Many other defects might
be mentioned, but these are sufficient to show that the manual system
is not perfect.
For a long time inventors have been striving to do away with all
such defects by abolishing the exchange operators, and substituting
mechanism to work the exchanges automatically, and during the last
few years the system of the Automatic Electric Company, of Chicago,
has been brought to great perfection. This system is in extensive use
in the United States, and is employed in two or three exchanges in
this country. Unfortunately the mechanism of this system is extremely
complicated, so that it is impossible to describe it fully in a book of
this kind; but some idea of the method of working may be given without
entering into technical details.
Each subscriber’s telephone instrument is fitted with a dial which
turns round on a pivot at its centre. This dial has a series of holes
round its circumference, numbered consecutively from 1 to 9, and 0.
Suppose now a subscriber wishes to speak to a friend whose telephone
number is 2583. He removes the receiver from its hook, places his
finger in the hole marked 2, and turns the dial round in a clockwise
direction until his finger comes in contact with a stop. He then
removes his finger, and the dial automatically returns to its original
position. He then places his finger in the hole marked 5, and again
turns the dial as far as the stop, and when the dial has returned
to the normal position he repeats the process with his finger placed
successively in the holes marked 8 and 3. He now places the receiver
to his ear, and by the time he has done this the automatic mechanism
at the exchange has made the necessary connexions, and has rung the
bell of subscriber number 2583. On completing the conversation each
subscriber returns his receiver to its hook, and the exchange mechanism
returns to its normal position.
The turning of the dial by the finger coils up a spring, and this
spring, acting along with a speed governor, makes the dial return to
its first position at a certain definite speed as soon as the finger
is removed. During this retrograde movement a switch automatically
sends out into the line a certain number of impulses, the number being
determined by the hole in which the finger is placed. In the case
supposed, groups of two, five, eight, and three impulses respectively
would be sent out, each group separated from the next by an interval
during which the subscriber is turning the dial.
Now let us see what takes place at the exchange. The subscriber’s
instrument is connected to a mechanical arrangement known as a “line
switch.” This switch is brought into play by the act of removing
the receiver from its hook, and it then automatically connects the
subscriber’s line to what is called a “first selector” switch. The
group of two impulses sent out by the first turning of the dial raises
this first selector two steps, and it then sweeps along a row of
contacts connected to “trunks” going to the 2000 section. Passing by
occupied trunks, it finds an idle one, and so connects the line to an
idle “second selector.” This selector is operated by the second group
of impulses, five in number, and after being raised five steps it
acts like the first selector, and finds an idle trunk leading to the
2500 section. This places the caller’s line in connexion with still
another switch called a “connector,” and this switch, operated by the
remaining groups of eight and three impulses, finds the required tens
section, and selects the third member of that section. If the number
2583 is disengaged, the connector switch now sends current from the
central battery to this instrument, thus ringing its bell, and it also
supplies speaking current to the two lines during the conversation,
restores the exchange mechanism to its original condition as soon
as the conversation is ended and the subscribers have hung up their
receivers, and registers the call on the calling subscriber’s meter. If
the connector finds the number engaged, it sends out an intermittent
buzzing sound, to inform the caller of the fact. All these operations
take time to describe, even in outline, but in practice they are
carried out with the utmost rapidity, each step in the connecting-up
process taking only a small fraction of a second.
For ordinary local calls the automatic system requires no operators
at all, but for the convenience of users there are usually two clerks
at the exchange, one to give any information required by subscribers,
and the other to record complaints regarding faulty working. For trunk
calls, the subscriber places his finger in the hole marked 0, and
gives the dial one turn. This connects him to an operator at the trunk
switchboard, who makes the required connexion and then calls him up in
the usual way.
It might be thought that the complex mechanism of an automatic exchange
would constantly be getting out of order, but it is found to work with
great smoothness. Each automatic switchboard has a skilled electrician
in attendance, and he is informed instantly of any faulty working
by means of supervisory lamps and other signals. Even without these
signals the attendant would be quickly aware of any breakdown, for his
ear becomes so accustomed to the sounds made by the apparatus during
the connecting-up, that any abnormal sound due to faulty connecting
attracts his attention at once. However detected, the faults are put
right immediately, and it often happens that a defective line is noted
and repaired before the subscriber knows that anything is wrong.
On account of its high speed in making connexions and disconnexions,
its absolute accuracy, and its privacy, the automatic telephone system
has proved most popular wherever it has been given a fair trial. Its
advantages are most obvious in large city exchanges where the traffic
during business hours is tremendously heavy, and it is probable that
before very long the automatic system will have replaced manual methods
for all such exchanges.
The telephone system is more highly developed in the United States than
in this country, and some of the exchanges have been made to do a great
deal more than simply transmit messages. For instance, in Chicago there
is a system by which a subscriber, on connecting himself to a special
circuit, is automatically informed of the correct time, by means of
phonographs, between the hours of 8 a.m. and 10 p.m. New York goes
further than this however, and has a regular system of news circulation
by telephone. According to _Electricity_, the daily programme is
as follows: “8 a.m., exact astronomical time; 8 to 9 a.m., weather
reports, London Stock Exchange news, special news item; 9 to 9.30
a.m., sales, amusements, business events; 9.45 to 10 a.m., personal
news, small notices; 10 to 10.30 a.m., New York Stock Exchange and
market news; 11.30 a.m. to 12 noon, local news, miscellaneous; 12 noon,
exact astronomical time, latest telegrams, military and parliamentary
news; 2 to 2.15 p.m., European cables; 1.15 to 2.30 p.m., Washington
news; 2.30 to 2.45 p.m., fashions, ladies’ news; 2.45 to 3.15 p.m.,
sporting and theatrical news; 3.15 to 3.30 p.m., closing news from Wall
Street; 3.30 to 5 p.m., musical news, recitals, etc.; 5 to 6 p.m.,
feuilleton sketches, literary news; 8 to 10.30 p.m., selected evening
performance--music, opera, recitations.” Considering the elaborate
nature of this scheme one might imagine that the subscription would be
high, but as a matter of fact it is only six shillings per month.
The telephone has proved of great value in mine rescue work, in
providing means of communication between the rescue party and those
in the rear. This end is achieved by means of a portable telephone,
but as the members of a rescue party often wear oxygen helmets, the
ordinary telephone mouthpiece is of no use. To overcome this difficulty
the transmitter is fastened round the throat. The vibrations of the
vocal cords pass through the wall of the throat, and thus operate the
transmitter. The receiver is fixed over one ear by means of suitable
head-gear, and the connecting wire is laid by the advancing rescuers.
A case containing some hundreds of feet of wire is strapped round
the waist, and as the wearer walks forward this wire pays itself out
automatically.
By the time that the telephone came to be a really practical
instrument, capable of communicating over long distances on land, the
Atlantic telegraph cable was in operation, and an attempt was made to
telephone from one continent to the other by means of it, but without
success. In speaking of submarine telegraphy in Chapter XVII. we saw
that the cable acts like a Leyden jar, and it was this fact that made
it impossible to telephone through more than about 20 miles of cable,
so that transatlantic telephony was quite out of the question. It was
evident that little progress could be made in this direction unless
some means could be devised for neutralizing this capacity effect, as
it is called, of the cable, and finally it was discovered that this
could be done by inserting at intervals along the cable a number of
coils of wire. These coils are known as “loading coils,” and a cable
provided with them is called a “loaded cable.” Such cables have been
laid across various narrow seas, such as between England and France,
and England and Ireland, and these have proved very successful for
telephonic communication. The problem of transatlantic telephony
however still remains to be solved. Experiments have been made in
submarine telephony over a bare iron cable, instead of the usual
insulated cable. Conversations have been carried on in this way without
difficulty between Seattle, Washington, U.S.A., and Vashon Island, a
total distance of about 11 miles, and it is possible that uninsulated
cables may play an extremely important part in the development of
submarine telephony.