This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
"Hello!" The answer comes back in a second. You don't know, perhaps, that as soon as you unhook the receiver, a tiny knob of light flashes out below your telephone number on the far-away switchboard of "Central." An operator, with a receiver strapped over her ears and a transmitter just below her mouth, sits before the switchboard. She is only one of dozens of young women operators. In front of each is an upright switch-board, or table on edge, that is punched as full of little round numbered holes as a honeycomb. Below each hole on the board, is a tiny glass knob no bigger than a shoe button. One of those little button lamps and one of those holes belong to your wire. When you ring up Central, your lamp flashes out on the board like a firefly. The operator sees it. She pushes a plug that carries a wire into that hole above the light, hears you and answers you.
"Give me Main 3908, please."
She pushes another plug on the end of a wire that connects with yours into the hole showing the number you called for. That rings the bell in the house of the friend you want to talk with.
"All right," calls the operator. "Put in your nickel, please."
"Hello! Is that you, Dick?"
Yes. You know his voice! And he knows yours, although you may be a mile, or ten or more miles apart. Then you have as nice a chat as if you were in the same room.
This wonderful thing is so common that we forget just how wonderful it is. Would you like to know about it?
Did you ever drop a pebble into a pool of still water? It makes a ring wave. Water rings widen and spread to the shores of the pond. Sound makes ring waves in the air. In a narrow valley these sound waves strike the rock walls and come back to you as echoes. It is these air waves that carry sounds to our ears (see Acoustics, Wave-Motion and Fairy Prince Echo), but they do not carry them very far. Men who have gone up in balloons say that a mile up in the sky, the only earth-sound they can hear is the whistle of a locomotive. How far away can you hear and recognize the voice of a friend?
Now there are electric waves as well as air, water and light waves. Electric waves travel fast and far. By striking the key of a telegraph instrument, in dots and dashes that stand for letter and words, messages are sent over wires charged with electricity, across wide lands and under wide oceans. These dots and dashes of sound are received just as they are sent. So telegraph operators know each other's ways of rattling off messages, just as you know the voices of many friends apart. It was long thought that electric wires would carry words and the very tones of the human voice, if a way could be found to get them on the wire. Of course, a spoken word cannot strike a key, as a finger can. But it can travel on an air wave, strike a rock wall and make an echo of itself.
" Make a wall, then, to catch air waves," was the idea the inventor of the telephone got. " But don't let the sound bounce back in echoes. Pass them on to an electric wire." The "wall" in the telephone, is a little round thin iron disc about as big as a penny, stretched as tight as a drum head. That is what you have in your ear—a drum head—to catch and pass on sound to the nerve of hearing. The auditory nerve is a sort of telephone wire to the brain.
This little iron drum head in the telephone connects with an electric wire. It catches the air waves made by your voice and passes them to the wire. On the electric waves the sounds travel with the speed of light to a drum-head disc in the receiver held at the ear of your friend. There the electric waves are changed back to air waves again, and your friend hears your words just as you speak them.
Isn't that wonderful?