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the information contained in this book is either outdated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
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
90 GILBERT BOY ENGINEERING
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
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
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-
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.
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
5. Do not temper with the magnet coil or other internal
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
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
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.
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.
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
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
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