ARTICLE OF THIS MONTH

                                                                  Telephone

       I can see your mouth laughing! Don't feel it as a silly topic. Though we are in the age of cell phone and WiFi , most of us don't know the basic concept of telephone. I  have published this article to clear the doubts regarding the telephone and its history and We hope that this article will provide you some worthy things. 

Introduction:

Telephone is an instrument that sends and receives voice messages, usually by means of electric current. (But the Hi tech telephones and cellular phones are capable of sending text and multimedia messages) 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.

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. The ear piece is nothing but a speaker that you can find in radio,television etc. The mouthpiece is a microphone (you might have seen it in churches) . Before making a call, the person listens in the earpiece for a dial tone.(But you cannot here it in a cellular phone of these days) 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 (microphone), the telephone converts the sound waves of the person's voice into an electric current. The telephone 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: 

(1) a dialling mechanism, 

(2) a transmitter 

 (3) a receiver. 

The dialling mechanism enables a caller to enter telephone numbers. The dialling 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 many modern telephones, the dialling mechanism consists of a set of buttons or keys called a keypad. A standard keypad has 12 keys--the digits 0 to 9, a "*" key, and a "#" key. When pressed, each key generates either a 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 transmitter, also called the microphone, converts the sound waves of a person's voice into an electric current and sends this current further into the telephone network. The transmitter is built into the handset, behind the mouthpiece. There are two main kinds of telephone transmitters--(1) the carbon transmitter and (2) the foil-electret microphone. 

The carbon transmitter has two main parts--a diaphragm and a carbon chamber. The diaphragm is a thin, round piece of aluminium. The carbon chamber lies behind the diaphragm, between two electric terminals. It contains many tiny grains of carbon. Low-voltage electric current travels through the carbon grains. A small, gold-plated brass dome located on the underside of the diaphragm also rests in the carbon chamber. 

When a person speaks into the mouthpiece, the sound waves cause the diaphragm to vibrate. A loud sound creates a strong vibration, and a soft sound a weak vibration. The motions of the diaphragm cause the dome to vibrate within the carbon chamber. Each vibration of the dome compresses the carbon grains. A small vibration places light pressure on the grains, while a large vibration squeezes them tightly together. The greater the compression of the grains, the more easily electric current passes through them. So the louder the sound, the greater the flow of electric current. 

The foil-electret microphone has a circular diaphragm consisting of a thin sheet of electrically charged plastic with a metal coating on one side. This diaphragm, called a foil electret, is stretched over a hollow metal disc called a backplate. The metal coating on the diaphragm faces away from the backplate. 

The diaphragm touches the backplate only in certain areas. In other areas, the diaphragm and the backplate are separated by air pockets. The diaphragm can vibrate in these pockets when hit by sound waves. 

The electric charge carried by the diaphragm sets up an electric field between the diaphragm and the backplate. The strength of this field depends in part on the distance between the diaphragm and the backplate. 

When a person speaks into the mouthpiece, sound waves hit the diaphragm. The resulting vibrations change the distance between the diaphragm and the backplate--and thus the strength of the electric field. These variations in field strength trigger corresponding variations in an electric current. 

The receiver converts the electric current coming through the telephone line into a duplicate of the speaker's voice. The receiver is built into the handset, behind the earpiece. 

In a receiver, an iron diaphragm in a flexible frame is surrounded by a ring-shaped permanent magnet. The permanent magnet exerts a constant pull on the diaphragm. Another magnet, called an electromagnet, is attached to the other side of the diaphragm. This magnet is made of metal with a coil of wire around it. 

When the electric current flows through the coil, the electromagnet becomes magnetized. The current moves in two directions. When it moves in one direction, the resulting magnetism adds to the pull of the permanent magnet and causes it to attract the diaphragm more strongly. When the current moves in the other direction, the resulting magnetism opposes that of the permanent magnet and reduces its pull on the diaphragm. The variation in magnetic pull causes the diaphragm to vibrate at the same rate. It pulls and pushes the air in front of it, producing pressure changes that create sound waves almost the same as those of the speaker's voice. The listener hears a duplicate of the speaker's voice.

 

                                   

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