THE TELEPHONE
IN the electric telegraph, the electric current is made to
turn a magnetic needle, or move an electric pen at a distance. When the
distance becomes very great, as in the telegraph cables which connect
us with America, a more delicate instrument is used for receiving the
messages. It consists of a kind of small ink bottle, which is turned
this way or that by the electric current. It squirts out a fine jet of
ink, and the ink falls on paper which is moved past the instrument. In
this way the signals are recorded on paper without any friction.
The electric telegraph, however, is a very simple and coarse instrument
in comparison with the telephone, for the telephone enables us to send
speech itself over several hundred miles, over distances which the most
powerful light from the highest lighthouse could not cover.
That any one can speak into an instrument in London, and that at the
same time a friend in Paris can hear his voice, and understand what he
is saying, is a thing so wonderful that a person must have tried it for
himself before he can quite believe it. Yet what is wonderful about it
is not so much the fact of speech being communicated, as the distance
across which the communication is made. Why should we not be able to
cover such great distances by shouting?
The reason is, that when we shout we create a wave of sound in the air.
Such a wave of sound is in many respects like a wave in water. When we
throw a stone into a pond, the wave starts from the place where the
stone fell into the water, and travels out in circles which become
wider and wider. Not only do the circles become wider, but they also
grow fainter and fainter the wider they get. If the pond is very large,
the wave may get so faint that when it reaches the bank it is too faint
to be perceived.
If, instead of a pond, we have a long channel or gutter filled with
water and make a splash in it at one end, we find that the wave can
travel a very great distance along the channel without growing much
feebler. That is because it cannot spread out sideways. Similarly, if
we speak into a long pipe or tube, our voice carries much farther than
it does in the open air, again because the wave of sound cannot spread
out sideways. This is the reason why it is so easy to talk through a
speaking-tube.
If we could send the same waves along a wire, we might succeed
in carrying the sound much farther than we do even through a
speaking-tube. This fact can be proved by taking two small tin cans,
and stretching a wire from a hole in the bottom of one to a hole in the
bottom of the other. Then, if one person speaks into one of the cans,
and the other person holds the other can to his ear, and the wire is
stretched between them, the second person will hear distinctly what the
first person is saying. In this case the wave of sound travels along
the wire, stretching it and compressing it in turn, until it reaches
the other can, and throws the air into similar vibrations. Such an
instrument could be called a telephone, but nowadays the word is used
only for instruments which are worked by electricity or magnetism.
To understand the difficulties which the inventors of the telephone
had to overcome, let us consider for a moment the nature of the sound
waves which represent speech. Spoken language consists of vibrations of
the air. These vibrations are very small and very quick puffs of air,
which we produce by means of our throat and mouth. Now if we were to
try to give a very rapid succession of puffs, we could not give more
than about ten per second. But if we had a machine which could puff
much more quickly than that, we should find that after the puffs had
become more than twenty per second, there would be a kind of hum. This
result can be illustrated by means of a toothed wheel such as we find
in clocks. When such a toothed wheel is turned quickly, and we hold the
edge of a piece of paper against it, then, as it is made to turn more
and more quickly, the little ticks of the paper merge more and more
into a hum, and finally become a musical note. The faster the wheel,
the higher the pitch of the note.
Every note has its own pitch, that is to say, it has a certain number
of vibrations per second. The notes in the middle of a piano have from
two hundred to five hundred vibrations per second. The same note may be
either loud or soft, just as a wave of water may be either high or low.
The high wave corresponds to a loud sound. Very small ripples on the
surface of water correspond to a very high, but faint, note of music,
like the singing of an insect.
Each note has its own pitch and its own loudness. In addition to that,
it also has a certain “quality” of its own. We may, for instance,
play the same note on a violin, a trumpet, or an organ. They may have
the same pitch and the same loudness, and yet we can distinguish them
from each other easily. What is this “quality” which enables us to
distinguish the note of one instrument from the same note played on
another instrument? It must be something in the position of the waves
which travel through the air and reach our ear.
This question long remained in doubt, until people found out that no
musical sound was quite simple. Even in water one usually sees little
ripples on the top of big waves. A wave with a perfectly smooth surface
is very rare. In the same way, a perfectly simple and pure sound wave
is hardly ever produced. Each wave has little ripples on it. The wave
from a piano has one kind of ripples, the wave from an organ has
another kind, and the wave from a trumpet--which is particularly shrill
and penetrating--has a great number of very tiny ripples, which mean
very high notes. These facts can be proved by playing a soft note on
an organ, and playing a number of very high notes at the same time.
If the high notes are properly chosen, the sound becomes shrill and
penetrating, like the sound of a trumpet.
Musical sounds are therefore quite fixed as soon as we know their
loudness, pitch, and quality. But not all sounds are musical. There
are such things as noises, and many forms of human speech are anything
but musical. As a matter of fact, speech consists of a very rapid
succession of all kinds of notes, which last only a very short time,
and rapidly change from one to the other.
Of these notes, the highest in pitch are the hissing sounds, such as
_s_ and _sh_, and these are most difficult to send through
the telephone. Shortly after the electric telegraph was invented, the
idea occurred to many electricians that it ought to be possible to send
speech along the wire conducting the electric current. They knew that
sound consisted of waves, and they thought that if an electric current
could be interrupted a great many times per second, a musical note
would be produced at the other end of the wire.
An observation made in America in the year 1837 confirmed this idea. It
was found that when a current is sent suddenly through a long coil of
wire wound round a piece of soft iron, a little sound is heard. Such a
coil is called an electromagnet, because the soft iron is a powerful
magnet so long as a current is passing through the coil. Another
sound is heard when the current is broken suddenly. This observation
suggested that if the electromagnet were at the other end of a long
wire, and the current at one end were made and broken rapidly, the
electromagnet would give a musical note at the other end.
In the year 1854, a French telegraphist proposed to send speech by
electricity on that principle. He proposed to stretch a skin over a
drum, and fix one end of a fine wire to the middle of the skin. The
end of the wire should dip into a drop of quicksilver connected with
another wire, and this contact should establish an electric circuit. On
speaking into the drum the skin would vibrate, the wire would move in
and out of the drop, and the electric circuit would be made and broken
in turn. It would be made and broken very rapidly when a high note was
sung into it, and less rapidly in the case of a low note. It would
follow all the various notes of ordinary speech, and would produce the
necessary vibrations at the receiving end.
The Frenchman never carried out his proposal. In the year 1861 it
was again taken up by a German professor of the name of Reis. His
instrument was able to transmit musical notes, but not spoken language.
It was a very wonderful instrument at the time, but it could not give
the correct quality to notes. The sudden induction of the electric
current was too jerky to give the very fine ripples on the sound waves,
and especially to render the delicate shades of human speech. The
inventor died poor and neglected in 1874, and it was not till 1876 that
the telephone problem was solved really.
The credit of the successful solution of this great problem is due to
an American of the name of Graham Bell. As is very often the case,
the successful instrument was extremely simple, so simple indeed that
people were disappointed on seeing it. It could be put together by a
schoolboy, and yet it was able to talk. From the very first, Bell had
seen the correct principle which must govern the construction of a good
telephone. The fault of Reis’s telephone was that it could not follow
the delicate wavelets of sound which constitute human speech. It was
necessary to make an instrument which should be extremely sensitive and
delicate, as much so as the human ear, which is saying a great deal. In
order to do this Bell used Faraday’s induction currents.
We have seen already that when Faraday pulled off the cap of the
electromagnet a momentary current was induced in the coil of wire.
Small currents could be induced by simply shaking the cap to and fro.
Now, Bell had the idea of mounting a small piece of iron in front
of the bar of an electromagnet. The ends of the coil of wire of the
electromagnet were pulled out to a very long distance and passed round
another electromagnet. The small piece of iron was stuck upon a piece
of parchment, which was stretched on a drum in front of the first
electromagnet. On speaking into this drum the parchment was set into
vibration, and with it the small piece of iron. This produced slight
and very rapid currents in the coil of the electromagnet, and these
were transmitted along the wire to the other electromagnet.
[Illustration: Fig. 58.--Graham Bell, inventor of the Telephone.]
Another parchment with a similar small piece of iron was mounted on
a drum in front of the second electromagnet, and it was found that
the second drum reproduced exactly all that was spoken into the first
drum. For the small currents arriving at the second electromagnet made
its magnetism stronger and weaker in rapid succession, and attracted
the iron sometimes more and sometimes less. Every motion of the first
parchment, therefore, was rendered faithfully by the second parchment,
and the vibrations of the air inside the first drum were reproduced
accurately in the second drum. That was all that was necessary for the
complete transmission of human speech.
But this apparatus was still capable of considerable improvement. In
the first place, we know that an electromagnet loses all its magnetism
when the current in its coil is stopped. But Graham Bell found that it
was not at all necessary to use an electromagnet.
[Illustration: Fig. 59.--Bell’s Magnetic Telephone.]
Instead of a piece of soft iron, he found he could use a similar piece
of steel which had been magnetised strongly. Such a piece of steel is
called a permanent magnet. The little horseshoe magnets with the red
paint on them that are sold in shops are such permanent magnets. This
discovery simplified matters greatly, as it was no longer necessary to
use a battery at all.
Another improvement was to do away with the parchment. Graham Bell
found that a sheet of tin would do very well instead of the parchment.
What is called “tin” is usually in reality a sheet of iron covered on
both sides with a very thin layer of metallic tin; and as the magnetism
acts quite easily through tin, the tinned iron acts like a sheet of
pure iron. When a sound wave reaches it, it is set vibrating; and
its vibrations slightly alter the magnetism of the permanent magnet,
and produce the small and rapid currents mentioned above. The whole
apparatus, therefore, consists simply of two steel magnets, two discs
of tin, sufficient wire to make coils round the magnets and cover the
distance between the two stations, and the necessary boxes to keep the
discs and magnets in the proper position. It is surely the simplest
instrument ever invented to perform such a wonderful task. What is
almost quite as wonderful is that this telephone is still in general
use for receiving telephone messages.
It says a great deal for the excellence of the instrument that
thirty-three years of active search for improvements have not been able
to suggest any real improvement in Graham Bell’s instrument. Bell’s
receiver is still used at the present day.
But his instrument is not as good for sending or transmitting speech as
some other instruments which have since been invented.
[Illustration: Fig. 60.—Carbon Transmitter.]
[Illustration: Fig. 61.--Combined Receiver and Transmitter.]
It was found that Bell’s transmitter was rather feeble as regards
loudness, and an improved instrument has been invented in America since
his time.
It was Edison, the inventor of the phonograph, who first proposed to
use carbon for sending the message. The new instrument was in some ways
a revival of the idea of Reis, but instead of breaking and making the
circuit, it was thought better to make the contact between the ends
of the circuit weaker and stronger in succession. This is now done
in an instrument called the Blake transmitter.
[Illustration: Fig. 62.--Microphone.]
An electric circuit containing a battery is interrupted at one
point, and joined again through a little ball of platinum and a ball
of carbon, such as is used in electric batteries. This contact is
connected with a vibrating plate which transmits the speech. The
circuit is never really broken, but is weakened and strengthened
alternately by a more or less perfect contact between the two balls.
The result is that human speech is conveyed with great clearness.
This principle of loose contact has since been employed for another
instrument which is perhaps even more wonderful than the telephone.
It is the “microphone,” which was first constructed by Edwin Hughes
in 1878. A current was sent through a Bell telephone and through two
pieces of carbon joined by a rod of carbon which was laid loosely
across them. The slightest sound produced on the board which held the
carbons was converted into a loud sound in the telephone. When a fly
walked across the board, the patter of its feet could be heard in the
telephone.