How does a telephone work?

All sound is made by the back and forth movement, or vibration, of objects. When an object vibrates, it makes the molecules around it vibrate too, causing a ripple of motion known as a sound wave, which can travel through air, water, and solid materials. A telephone has vibrating parts a disc in the mouthpiece and one in the earpiece that turn voice sounds into electrical signals that can travel along telephone wires and then turn back into sound again.

Phones are usually connected to wires because they run on electricity. When you pick up a phone, a low electrical current allows you to dial the series of numbers that will connect you to the phone of a friend, for example. Each number on the phone has its own special electrical signal, and when the right numbers are combined, they can exactly identify your friend’s phone line. A local telephone office receives this information when you dial and sends your call in the right direction. Call signals to places close by travel along wires or cables buried underground or strung high in the air between supports. But when the person you call is very far away, the electrical signals sent from your phone are changed into invisible waves of energy called microwaves, which can travel long distances through the air. These waves are sent through space to communication satellites that orbit the world, which bounce them back to Earth, extending the waves’ travel distance. An antenna at a receiving station near the home of your friend picks up the waves and changes them to electrical signals once more. They travel by cable to the telephone office that services your friend’s neighborhood, where his or her number is identified. A signal is then sent to your friend’s phone to make it ring. The entire process from dialing to ringing takes just a few seconds!

When your friend answers the phone, a microphone in its mouthpiece contains a plastic disc that vibrates, turning his or her message into electrical signals that travel along the same path as before. A speaker in the earpiece of your phone receives the electrical signals, which vibrate another plastic disc that changes them back into sound. Two circuits from microphone to speaker are created, and you can talk back and forth with your friend. The next time you use the phone, think of the remarkable process that makes it possible!