The
Science Notebook
Gilbert Sound - Chapter VIII
NOTE:
This book was
published around 1920 as a manual to accompany the Gilbert
Sound set. The set and manual were part of the "Boy
Engineering" series, While some of the
experiments and activities here may be safely done as written, some of
them may be considered hazardous in today's world. In addition,
some of the
information contained in this book is either outdated
or inaccurate. Therefore, this book is probably best
appreciated for its historical value rather than as a source of
current information and good experiments. If you try
anything here, please understand that you do so at
your own risk.
See our Terms
of Use.
Pages 89-End
Chapter VIII
MODERN INVENTIONS
THE TELEPHONE. There
is nothing finer in the history of modern inventions than the story of
the invention of the telephone. It demonstrates so well fact that
none of the important inventions in electricity have been stumbled upon
by chance, as many suppose, but they are all the result of painstaking
experiments and tireless research by men of vision and
determination. This story is one which often comes to me when
puzzling over new problems in the field of science.
You
may be surprised to learn that Alexander Graham Bell, inventor of the
telephone, was not much of an electrician. He was, however, at
the head of his profession as a teacher of vocal expression and was, at
a very early age, an authority on the nature of sound. Indeed, it
was through Bell's efforts to help deaf people hear, that he gained the
knowledge which enabled him to invent the telephone.
For
three years before the first telephone talked, Bell made an exhaustive
analysis of sound waves and their action on the ear drum. He made
records of various vibrations of the human voice, after the manner
described in Experiment No. 12, except in a more elaborate way.
After
two and a half years of such research, Bell began to build the
apparatus which was to transmit sound vibrations by means of
electricity. He very soon realized that his electrical knowledge
was not equal to the undertaking, and therefore gave himself up to a
study of electricity. He sought out the
(89)
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men
who had had a large part in perfecting the telegraph, for he saw that
his problems, so far as electricity was concerned, were much the same
as theirs had been.
In less than a year after beginning
his intensive study of electric currents, Bell succeeded in producing
an instrument by which the vibrations due to sound waves caused a
delicate electric circuit to be made and broken in such a way that,
acting upon an electro-magnet at the other end of the wire, these
vibrations could be accurately reproduced. Thus, in the spring of
1876, the telephone was born, and the principle of that first crude
instrument has never been changed.
In the meantime,
other men, more learned in electrical science, had seen the possibility
of perfecting a telephone, but their attempts were failures because
they did not know what Bell knew about the nature of sound. In
the years following the birth of the telephone strong efforts were made
by others to claim credit for its invention, but the test of time has
proved that no modern inventor is more deserving of the fame he has won
as Alexander Graham Bell.
You
may easily examine a telephone transmitter and receiver and trace the
important units. (Refer to Figure 56.) By unscrewing the
transmitter cup and cap (which may be compared to the outer ear) you
will see a large, thin disc. This is connected by a short rod
(corresponding to the bones of the middle ear) to a diaphragm over the
flat side of a small semi-sphere, filled with carbon granules (the
nerve chamber of the telephone). You will notice, by tracing the
wiring, that the current passes through this cup or semi-sphere.
As the large disc is vibrated by air waves of the voice, it rapidly
changes the pressure on the carbon granules within the cup. These
changes in pressure vary the resistance of the carbon and hence vary
the strength of the current passing through the circuit.
By unscrewing the receiver cup, you will see a simple electro-
GILBERT SOUND EXPERIMENTS 91
magnet
directly behind a large disc (similar to the one in the
transmitter). The strength of this magnet is changed rapidly by
the changes in current passing around it. As the magnet changes
in strength, the large disc is first attracted, then released, and in
this way caused vibrate in exactly the same way as the transmitter
disc.
You can connect up the receiver and transmitter,
as in Figure 56, using only one dry cell, and have a lot of fun and
interesting study of the telephone in its simplest form.
MAKING CONNECTIONS
1.
Whenever connections are to be made the wires must be scraped off for
about an inch until the clean, bright copper shows. In making
connections be sure that the wire, at the point of contact, is clean
and bright and that all binding posts are clean and screwed down
tightly against the wire.
To connect two (2) wires,
twist the wire securely together, making sure that all insulation is
scraped back. A loose connection will prevent the set from
working.
2. If the receiver sounds rattly and
makes scratching sounds, look at the receiver and transmitter
diaphragms and see that they are held tightly in place by the receiver
caps or mouthpiece as the case may be.
3. Be sure
that every connection is perfect. A single broken wire or bad
contact will not permit the current to pass, and thus the instruments
will be inoperative. Poor electrical connections always cause
trouble.
4. When two or more batteries are used,
be sure that the center post on one is connected to the edge post of
the other. Otherwise, the batteries will "buck" each other and no
current will pass.
5. Do not temper with the magnet coil or other internal
92 GILBERT BOY ENGINEERING
parts of the receiver or transmitter. Be careful not to bend or dent the diaphragm.
6.
Examine the diaphragms and, should any be slightly concave, turn it
over so that the concave side is downward. If the concave side is
up, it is liable to prevent the set from working.
When
the instruments are properly set up, close the doors between the rooms
so the the natural voice cannot be heard, and after switching on the
battery current talk distinctly into the transmitter with the mouth
squarely facing it and about 3 or 4 inches away.
HINTS
In the event that the set will not work, although these instructions have been carefully followed, make the following tests:
1.
Connect the batteries in series as shown in the illustration, (see
Figure 58), then attach the outside wires to each other. If the
batteries are in good condition, there will be a very slight spark
given off when the wires are first touched together or pulled
apart.
2. If the batteries are in good condition,
attach one (1) wire of a receiver to one (1) of the battery wires, then
attach the other receiver wire to the remaining battery wire.
Should the receiver prove in good condition, you will note a ticking
noise each time the wires are touched together. Try the other
receiver in the same way and, if you do not hear the ticking noise, you
will know that the receivers are imperfect and will not work.
3.
Next connect one (1) receiver and one (1) of the transmitters, as shown
in Figure 56, to the battery. Place the receiver at the ear, then
blow against the transmitter diaphragm. If the transmitter is all
right, you should easily hear the sound of your breath through the
receiver.
GILBERT SOUND EXPERIMENTS 93
Another
test would be to shake the transmitter, still listening in the
receiver. If the circuit is O.K., you should hear a frying or
hissing noise, due to the rapid change of resistance of the carbon
granules in the transmitter.
Should any of these parts
test unsatisfactory to the previous tests, examine your wiring and
connections for it is evident that either you have a broken wire or
loose connection in your circuit.
THE PHONOGRAPH.
You probably know a lot about the machinery of a phonograph, but the
did you ever stop to think what a great variety of the principles of
sound are embodied in it?
When making a record, a blank
cylinder or disc of wax is placed in the machine and a sharp pointed
recorder is placed where the needle ordinarily is. The sound
waves from the singer or musical instruments are picked up by the
horn and carried to a small disc in the recorder, which is set into
forced vibration. The recording point, being attached to this
disc, moves up and down and, as the wax plate or cylinder revolves,
cuts an irregular groove in the wax. The depth and frequency of
the indentations within the spiral groove on the record correspond
exactly with the amplitude and frequency of the vibrations set up by
the music or voice that is being recorded.
When the
record is played, a reproducer is substituted for the recorder.
The main difference between the two is that the needle used with the
reproducer is not nearly so sharp as the point of the recorder.
The needle simply follows the tiny indentations of the spiral groove
and reproduces the original vibrations.
This does not complete
the story of the phonograph, however, because the vibrations reproduced
without amplifying and resonating devices are of little value as music,
just as the violin string without the violin is a very ordinary
thing. You may prove this fact for yourself in a manner that is
most interesting and conclusive.
Experiment No. 42. Remove an eraser from a pencil and
94 GILBERT BOY ENGINEERING
Force
a pin through it in such a way that, when you replace it in the pencil,
the point of the pin will project out, as in Figure 59. Put a
record on the phonograph and start it revolving. With a pencil
held between the teeth, let the pin point follow the spiral groove in
the record. To make the experiment more effective, put your
fingers to your ears, shutting out all sounds from the room. You
will be able to hear the words of the record very plainly, yet other
persons in the room will hear nothing at all.
The
vibrations caused by the record in this case are transmitted through
the pencil to the teeth, and from there through the bones of the skull
to the tiny bones of the middle ear, which in turn pass them on to the
auditory nerves as described above. The other persons in the room
hear nothing because the vibrating body is so small that it does not
produce enough volume of sound to be transmitted through the air.
By means of a reproducer, the important part of which is a
vibrating disc, the sounds are magnified according to the principle
that the greater the area of the vibrating surface the more intense
will be the sound produced. From the reproducer the sound is
carried through a tube to the horn or sounding box. This is
simply a resonator and is designed to bring out the quality of the
tones. There are numerous types of phonographs manufactured, but
the difference between the good ones and the cheap ones may be found in
the way in which their reproducing and resonating devices bring out the
overtones - that is, the quality of the original tones.
[Adertisements]
