More Inventions (IV)

The telephone

Telephone is an instrument that sends and receives voice messages, usually by means of electric current. It is one of our most valuable means of communication. In just a few seconds, you can telephone a person across the street, in another part of the country, or on another continent. The word telephone comes from two Greek words meaning far and sound. 

In its most basic form, a telephone enables people to talk with one another at distances beyond the range of the human voice. More sophisticated telephones can send and receive not only voice messages, but also written words, drawings, photographs, and even video images. In addition, telephones can send information from one computer to another. 

Telephones in people's homes are connected through a vast, complex telephone network. The network includes large computers, tremendous lengths of copper wire and hair-thin strands of glass, cables buried in the ground and laid along the bottom of the oceans, radio transmitters and receivers, and artificial satellites orbiting far above the earth. 

Most telephones connect with the telephone network by means of wires that run through the walls of houses and other buildings. A small clip connects each telephone to the wiring. Other phones are installed in cars or carried in a bag or pocket. Such phones connect with the network by radio. 

Alexander Graham Bell patented the telephone in 1876. Today, hundreds of millions of telephones serve people all over the world.


How a telephone works

The part of a telephone that a person picks up to make a phone call is the handset. It has an earpiece and a mouthpiece. Before making a call, the person listens in the earpiece for a dial tone. This sound indicates that a telephone line is available to handle the call. The caller then enters a telephone number that specifies the telephone being called. The telephone network uses the numbers to establish a connection between the two phones. When the caller speaks into the mouthpiece, the telephone converts the sound waves of the person's voice into an electric current. The network uses various devices to create an almost perfect duplicate of the current in the telephone of the person being spoken to. That telephone converts the current into sound waves very much like those of the caller's voice. 

A telephone has three main parts:

• A dialing mechanism 
• A transmitter 
• A receiver. 

The dialing mechanism enables a caller to enter telephone numbers. The dialing mechanism may be built into the handset, between the earpiece and the mouthpiece. Or it may be part of a separate base unit that connects by cord to the handset. 

In most telephones manufactured since the 1960's, the dialing mechanism consists of a set of buttons or keys called a keypad. A standard keypad has 12 keys--the digits 0 through 9, a "*" key, and a "#" key. When pressed, each key generates either a certain number of electric pulses or a pair of accurately controlled tones. Computers in the telephone network use the sequence of pulses or tones to direct the call. 

The dialing mechanism on some telephones consists of a disk called a rotary dial. The dial has finger holes that correspond to the digits 0 through 9. A caller enters a telephone number by rotating and releasing the dial. This action generates electric pulses. 

The transmitter, also called the microphone, converts the sound waves of a person's voice into an electric current and sends this current farther into the telephone network. The transmitter is built into the handset, behind the mouthpiece. 

There are two main kinds of telephone transmitters :

• the carbon transmitter 
• the foil-electret condenser microphone.

For more information, see cellular phone.