Year: 1878




AN instrument to convey sounds BY
means of electricity" That gives one a
general idea of it; but, after all, that an.
swer is not the right one. The telephone
does not convey sound.


What does its name mean, then ?" do
you ask?


Simply, that it is far sounder but that
does not necessarily imply that il carries
sounds aiar. Strictly speaking, the tele-
phone only changes sound.waves into waves
of electricity and back again.


The most of you probably know that
sound is produced by rapid motion. Put
your finger on a piano wire that is sound
ing, and you will feel the motion, or touch
your front tooth with tuning fork that ia
singing; in the last case you will feel very
distinctly the raps made by the vibrating
fork. Now, sounding body will not only
jar another body which touches it, but it
Will also give its motion to the air that
touches 1T; and when the ar-motions or
air waves strike the sensitive drums of our
ears, these vibrate, and we hear the sound.


Thus, from our every-day experience we
have proof of two important facts, first.
sound is caused by rapid motion second
sound-waves give rise to corresponding mo.
tlon. Both these facts are involved in the
speaking telephone, which performs a two.
fold office, that of the air on the one hand,
that of our vocal organs on the other.


To serve as an ear, the telephone must be
able to take up quickly and nicely the sound
waves OF the air. A tghtened drum-head
Will do that; or better, a strip of goldbeat-
ers''skin drawn tightly over ring or the
end of tube. But Professor Bell wanted
an ear that would translate the waves of
sound into waves of electricity.


Just when Mr.. Bell was thinking about
this, some one experimenting with mag
net having coil of silk covered wire around
1t, found that when piece of iron was NOV.
ed in front of the magnet and close to it
without touching, the motion would give
rise to electric waves in the coil of wire
which waves could be sent great distance
along wires.


This was just what Mr.. Bell wanted. He
said to himself, The sound of my voice will
give motion to thin plate of iron as well
as to sheet of goldbeaters' skin; and if /
bring this vibrating plate of iron close to q
magnet, the motion will set up in it waves
of electricity answering exactly to the sound
waves which move the iron plate"


But the instrument must not only trans
late sound waves into electric impulses it
must also change these back again into
sound-waves; it must not only hear, but
also speak


You remember our first fact in regard to
sound: it is caused by motion. All that is
needed to make anything speak is to cause
1t to move so as to give rise to just such air
waves as the voice makes. Mr.. Bells idea
was to make the iron plate of his sound re-.
ceiver speak.


he reasoned in this way !. From the ma.
ture of the magnet it follows that when
waves of electricity are passed through the
wire coil around the magnet the strength of
the magnet must vary with the force of the
electric impulses. Its pull on the plate of
iron near it must vary in the same manner.
The varying pull on the plate must make it
move, and this movement must se' in mo
ton the air near the plate in sound waves
corresponding exactly with the motion set
ting up the electric waves in the first place
in other words, the sound motion in one
telephone must be exactly reproduced as
sound waves in similar instrument ioined
to it by wire.


Experiment proved the reasoning correct;
and thus the speaking telephone was invent
ed


The receiving and sending instrument ts
are precisely alike, each answer for
both purposes but there must be two since
one must always be hearing while the other
is speaking.


When you speak into the mouth piece of
Bells telephone, the sound of your voice
causes the Diaphragm to yibrate in front
of the magnet, The vibrations cause the
magnets pull upon the diaphragm to vary
IN force, which variation is answered by
electrical waves in the coil and over the
wires connected with it, At the other end
of the wire the pull of the magnet OF the
speaking telel hone is varied exactly in pro
portion to the strength of the electric i'm
pulses that come over the wire the varying
pull OF the magnet sets the diaphragm iu
motion, and that sets the air in motion in
waves precisely like those of the distant
voice. When those waves strike the list
ner's ear, he seems TO hear the speakers
exact tones, aud so, substantially, he does
hear them.-&. Nicholas for June