Part 17

A month later an enterprising Boston man put up a crude switchboard in his office and connected up five banks, using the system for telephoning in the day-time and as a protection against burglars at night. This was the beginning of the exchange system, all previous telephoning having been between two parties on the same circuit.

~NINE MILLION TELEPHONES IN U. S.~

Soon after exchanges sprang up in several cities, and by August of that year there were 778 Bell telephones in use. From this modest beginning the telephone has grown until on January 1, 1914, there were 13,500,000 telephones in the world, nearly 9,000,000, or over 64 per cent being in the United States.

When the wires come to this frame they are in numbered order in the cable. The main frame redistributes these wires so that they are arranged according to their call numbers, making it possible to connect any wire with any other wire anywhere that telephone service is installed.]

Breaking Up the Asphalt Pavement. First Step in Laying an Underground Cable.]

The use of the telephone instrument is common, but it affords no idea of the magnitude of the mechanical equipment by which it is made effective.

~UNSEEN FORCES BEHIND YOUR TELEPHONE~

To give you some conception of the great number of persons and the enormous quantity of materials required to maintain an always-efficient service, various comparisons are here presented.

The poles are set all over this country, and strung with wires and cables; the conduits are buried under the great cities; the telephones are installed in separate homes and offices; the switchboards housed, connected and supplemented with other machinery, and the whole system kept in running order so that each subscriber may talk at any time, anywhere.

Where Does Sound Come From?

Somebody or something causes every sound we hear. Sounds are the result of disturbances in the air. Sound is produced by waves in the air. The buzz of the bumble-bee is caused by the quick movement of his wings in the air. The wings themselves do not make the sound, but their motion causes waves or vibrations in the air which produce the sound of buzzing. Every motion made by anybody or anything produces waves in the air just like the waves you see in the water--a big movement makes a big wave and a tiny movement a tiny wave. When you clap your hands you make a disturbance in the air which causes a sound--the harder you clap the louder the sound. You can hear this sound and anybody else near can hear it. If there were no air about us, however, we would hear no sound, even if we could live in such a condition of things, for it is the air waves produced striking against the drum of our ears that enable us to discern sounds. When we talk we make air waves also and thus produce sound. If you were deaf, and talked, you could not hear any sound, because even when there are air waves they must still strike against a sounding board in order to be recognized as sound--and the drum of our ear is our sounding board for hearing sounds.

When the air waves produced are regular we call the sound musical, and when they are irregular we call it noise. Some people can make musical sounds when they sing, while others cannot.

If you take a piece of thin wire and stretch it tightly, fastening it at both ends, and then pull it with your finger and let go, you will hear a musical sound, because the vibrations produced will be regular and will continue for some time. If you shorten the distance on the wire where it is fastened at both ends and pull it as before, the sound produced will be in a higher key. If you take a guitar and snap the big G string you will produce the bass note of G. If the other G string (the smaller one) is in tune (if you watch the smaller one closely while you strike the larger one) you will notice the smaller one vibrate also. Sound waves of the same tone, although in different octaves, produce the same sounds, although in different keys.

This is the principle on which the piano is made to produce music. Inside the piano are wires of different lengths and the keys of the piano are arranged to operate certain little hammers, each of which strikes a certain wire. Every time you strike a piano key you cause one of the little hammers to hit its wire--the wire then makes vibrations which cause air waves. The air waves strike against the sounding board which is located behind the wires, and being thrown back into the air, strike against the drum of our ears, and we can hear the note.

Why Can We Make Sounds With Our Throats?

The sounds we make when we talk are produced in exactly the same way with the exception of the little hammers. In our throats are two cords which we call our vocal cords. When we talk we cause these cords to vibrate and thus we make the sounds of our voices. The most wonderful part of this voice of ours is that with only two vocal cords or wires, we can produce practically all the notes that can be made with a piano, which has a wire or cord for every note, excepting that we cannot make so many at one time. The human throat is so wonderfully constructed that we can lengthen or shorten our vocal cords at will and produce, with two strings, in our throats as many notes as it takes the piano many more strings to produce.

Why Does the Sound Stop When We Touch a Gong that Has Been Sounded?

When we touch the gong we stop the sound waves which the gong gives off when it is struck. These sound waves continue after the gong has been struck in continuous vibrations until something stops them. When you touch the vibrating gong, you stop its vibrating. If you only touch your finger to the vibrating gong you can feel the vibrations which cause a little tickling sensation. Naturally when you stop these vibrations you stop the air waves which the vibrations cause, and thus also the sound of these air waves striking your ear are stopped and the sound ceases.

How Can Sound Come Through a Thick Wall?

A sound will come through a thick or thin wall only if the wall is a good conductor of sound. Some things are good conductors of sound and others are not, just as some things are good conductors of electricity and others are not. If a wall is built of materials all of which are good conductors of sound, the sound will come through it no matter how thick. Wood is an especially good conductor of sound. It is even better than air. You can stand at one end of a long log and have another person at the other end hold up his watch in the air, and you cannot hear the watch tick, but if the watch is “going” as we say, and you ask the person holding it to put the watch against his end of the log, and you then put your ear to the other end, you can hear the watch ticking almost as well as if you had it to your own ear. In like manner you can hear the scratching of a pin at the other end of the log. When you put your ear against a telegraph pole you can hear the hum of the wires while you cannot hear it through the air. All sound is produced by sound waves and many solids are better conductors of sound waves than the air.

Sound waves, however, will sometimes not be heard as plainly through a wall, because of the fact that the wall may be made of materials which are not equally good conductors of sound. When a sound wave strikes a poor conductor it loses some of its power and the sound, although it may be heard through the wall, will be fainter.

What Is Meant by Deadening a Floor or a Wall?

By deadening a floor, for instance, we mean inserting between the ceiling of the room below and the floor above, or in the instance of a deadened wall, between the two sides of the wall, some substance like felt, paper or other non-conductor of sound, which will prevent the sound waves from passing through. This deadens them to the passing of sound or makes them sound-proof.

What Makes the Sounds Like Waves in a Sea Shell?

The sounds we hear when we hold a sea shell to the ear are not really the sound of the sea waves. We have come to imagine that they are because they sound like the waves of the sea, and knowledge that the shell originally came from the sea helps us to this conclusion very easily.

What Are the Sounds We Hear in a Shell?

The sounds we hear in the sea shell are really air waves or sounds made by air waves, because all sounds are produced by air waves.

The reason you can hear these sounds in a sea shell is because the shell is so constructed that it forms a natural sounding box. The wooden part of a guitar, zither or violin is a sounding box. They have the faculty of picking up sounds and making them stronger. We call them “resonators,” because they make sounds resound. The construction of a sea shell makes an almost perfect resonator. A perfect resonator will pick up sounds which the human ear cannot hear at all and magnify them so that if you hold a resonator to the ear you can hear sounds you could not otherwise hear. Ear trumpets for the deaf are built upon this principle.