Telephones

November 24, 2012 1:35 pm

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Telephones
What if you want to talk right now to a friend who lives far away? The answer is simple. You pick up your telephone and press some buttons. Next, you hear a ringing sound—one, two, three rings. Then you hear your friend’s voice say, “Hello.” Making a phone call seems so easy. But did you ever think about what makes it possible?
When you pick up your phone, it instantly hooks up with a vast, worldwide telephone network. The network has millions of miles of wire. It has cables that run under the oceans. It has optical (glass) fibers as thin as a hair. It has satellites that orbit high above Earth. It has powerful computers that keep track of everything on the network, including the call to your friend. The word “hello” might have zipped through wires, shot up to a satellite, or zoomed through a cable under the sea before it got to your ear.
HOW DOES YOUR TELEPHONE WORK?
Take a close look at your telephone. It has several parts. The part that you speak into is called the transmitter. The part that you put to your ear for listening is called the receiver. The buttons you press are called the dial.
Most phones have a handset and a base. The handset contains the transmitter and receiver and sometimes the dial. We often call the entire handset the receiver, even though only the part we place to our ear receives sound. Many phones have a wire that connects the handset to the base. Wires connect the base to the telephone network.
Some phones use radio signals instead of wires to send messages. Radio signals connect the handset of a cordless phone to the base. Cordless phones let you walk around while talking, but you can’t go too far. Radio signals from a cordless phone only work over a short distance.
Radio signals from a cell phone go much farther. A cell phone does not need a base to connect to the telephone network. You can take your cell phone almost anywhere. Radio signals from a cell phone go to an antenna, a tower that picks up radio signals. The area around an antenna is called a cell. The antenna in one cell can send signals to an antenna in another cell. An antenna can also send radio signals to wires in the telephone network, enabling you to call anyone you want.
HOW DOES YOUR VOICE TRAVEL?
Your voice does not really go through the telephone wires. Instead, a copy of your voice goes to the telephone network. The copy can travel through wires. It can also go through the air as a radio signal.
The telephone’s transmitter makes the copy of your voice. When you say “hello,” sound waves go into the transmitter. The sound waves hit a thin sheet of metal or plastic inside the transmitter. The sound waves make the sheet vibrate. Some vibrations are big and some are small. The transmitter turns the vibrations into electric signals. These signals are a copy of your voice saying “hello.”
The copy of your voice goes through the telephone network and into the receiver in your friend’s telephone. The way the receiver works is the opposite of the way the transmitter works. The receiver turns the electric signals into sound waves. Your friend hears the copy of your voice say “hello.” It sounds just like you!
HOW DOES YOUR FRIEND GET THE CALL?
Millions and millions of phones are connected to the telephone network. Computers and other equipment in the network can tell which phone belongs to your friend by the telephone number you dial. A telephone number is a kind of code. All phones in the United States have a code that is ten numbers long. The numbers are also called digits.
The first three digits of a phone number are called the area code. An area code tells the network what part of a state or city the phone is in. The next three digits are the exchange. They tell what neighborhood or other small area the phone is in. The last four digits tell the network exactly which phone in that area and exchange belongs to your friend.
Phone numbers in other countries have different numbers of digits. You also have to use extra numbers, called a country code, to call a different country.
BEFORE THE TELEPHONE
Before the telephone, it was hard for people to communicate over long distances. They wrote letters to each other. It could take days or even weeks for letters to be delivered.
Then people learned how to send telegraph messages. The messages traveled as electric signals that represented a code of dots and dashes. An operator on the other end converted the dots and dashes into a regular message. As the telephone became more and more popular, it largely replaced the telegraph.
WHAT CAN A TELEPHONE NETWORK DO?
Today our huge telephone network does many things besides carrying telephone calls. It sends copies of letters and pictures from one machine to another, called a fax machine. It connects computers all over the world into another vast network called the Internet. This network lets you send e-mail messages from your computer to your friends’ computers. It is hard to imagine what life would be like without the telephone.
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