Mark Cline, age 9, of Marion, N.C., for his question:
HOW IS A TELEPHONE CALL COMPLETED?
Almost 1 million men and women are employed in the telephone industry in the United States and Canada. Best known employee, perhaps, is the telephone operator who helps complete calls, but there are also lots of workers in the office where billing is handled and in the factories where the equipment is made. The industry also uses lots of installers and researchers.
Ever since Alexander Graham Bell said " Mr. Watson, come here. I want you!'' in his first phone call on March 10, 1876, the telephone has grown to be the most important unit in the world's communication system. More than 290 million telephones serve people all over the world today, with about 125 million being used in the United States.
The telephone uses electricity to carry sound. When you speak into the mouthpiece of a phone, the waves created by your voice strike a diaphragm in the transmitter and make it vibrate. A small cup filled with tiny grains of carbon is located in the diaphragm. The pressure of the sound waves on the carbon grain regulates the amount of electric current flowing through them. As the current becomes weak or strong, it copies exactly the wave pattern of the sound waves.
A pair of wires extends from the telephone of a person making a call to switching equipment in a central office. A trunk line carries the call to switching equipment in the central office that serves the phone being called. Automatic switching equipment can connect any two telephones in just seconds.
A vast network of wires crisscrosses North America. About 98 out of every 100 telephone wires are bound together in cables, with more than 4,000 wires in some of the cables.
Wires and cables are strung on telephone poles or placed underground. On many long distance calls, the telephone company's automatic switching system sends the call by radio waves by way of a radio relay system.
Radio relay systems use waves called microwaves. They travel mostly in straight paths and must be focused from one relay station to another, with each being about 30 miles apart. A microwave route can carry about 23,000 telephone conversations.
More than 99 percent of the telephones in the United States are dial operated and switched automatically. To complete a call from one side of the continent to the other, or from one side of the street to the other, all you have to do in most cases is dial the right set of numbers.
Communications satellites, orbiting the earth at 22,300 miles above the Atlantic and at the equator, appear to remain in one spot so that telephone messages, relayed from earth can be bounced back to some distant relay station.
Laser beams may someday carry vast amounts of our communications. And television equipped telephones that permit both speaking parties to see each other are now available for conference use.