The Science Notebook
Gilbert Sound - Chapter VIII

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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  In addition, some of the information contained in this book is either outdated or inaccurate.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

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



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-


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 pass. 

5.  Do not temper with the magnet coil or other internal


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. 


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. 


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


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.


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