SOUND WAVES AND THE TELEPHONE
"Hello, central!"
"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 letters
