Hi guys.. this is my first posting. This blog is dedicated to all technology lovers..
SO, here i go with the first one,..... the very common... telephone!!
A) HISTORY OF TELEPHONE
A number of inventors believed voice signal might be carried over wires, and all worked toward this end. The first to achieve success was a Scottish-born American inventor, Alexander Graham Bell, a speech teacher in Boston, Massachusetts.
Bell believed that voices could be reproduced at a distance, with the help of a transmitter and a receiver. On March 10, 1876, he and his assistant, Thomas A. Watson , prepared to test the mechanism. Accidentally, Bell spilled some acid on himself. In another room, Watson, next to the receiver, heard clearly the first telephone message: “Mr. Watson, come here; I want you.”
A few hours after Bell had patented his invention, another American inventor, Elisha Gray, filed a document called a caveat with the U.S. Patent Office, announcing that he was well on his way to inventing a telephone. Other inventors, such as Amos E. Dolbear , also made claim to having invented the telephone at the same time. Lawsuits were filed by various individuals, and Bell's claim to being the inventor of the first telephone had to be defended in court 600 times before the Supreme Court of the United States decided in his favour.
B) INTRODUCTION
Telephone, instrument that sends and receives voice messages and data. Telephones convert speech and data to electrical energy, which is sent great distances. All telephones are linked by complex switching systems called central offices or exchanges, which establish the pathway for information to travel. Telephones are used for casual conversations, to conduct business, and to summon help. The telephone has other uses that do not involve one person talking to another, including paying bills (the caller uses the telephone to communicate with a bank's distant computer) and retrieving messages from an answering machine. In 1999 there were 682 main telephone lines per 1,000 people in the United States and 655 main telephone lines per 1,000 people in Canada.
About half of the information passing through telephone lines occurs entirely between special-purpose telephones, such as computers with modems. A modem converts the digital bits of a computer's output to an audio tone, which is then converted to an electrical signal and passed over telephone lines to be decoded by a modem attached to a computer at the receiving end. Another special-purpose telephone is a facsimile machine, or fax machine, which produces a duplicate of a document at a distant point.
Now, there are many parts in telephone. lets look at them in brief....
C) PARTS OF TELEPHONE
1) TRANSMITTER:
There are two common kinds of telephone transmitters: the carbon transmitter and the electret transmitter. The carbon transmitter is constructed by placing carbon granules between metal plates called electrodes. One of the metal plates is a thin diaphragm that takes variations in pressure caused by sound waves and transmits these variations to the carbon granules. The electrodes conduct electricity that flows through the carbon. Variations in pressure caused by sound waves hitting the diaphragm cause the electrical resistance of the carbon to vary—when the grains are squeezed together, they conduct electricity more easily; and when they are far apart, they conduct electricity less efficiently. The resultant current varies with the sound-wave pressure applied to the transmitter.
The electret transmitter is composed of a thin disk of metal-coated plastic and a thicker, hollow metal disk. In the handset, the plastic disk is held slightly above most of the metal disk. The plastic disk is electrically charged, and an electric field is created in the space where the disks do not touch. Sound waves from the caller's voice cause the plastic disk to vibrate, which changes the distance between the disks, and so changes the intensity of the electric field between them. The variations in the electric field are translated into variations of electric current, which travels across telephone lines. An amplifier using transistors is needed with an electret transmitter to obtain sufficiently strong variations of electric current.
2) RECEIVER:
The receiver of a telephone set is made from a flat ring of magnetic material with a short cuff of the same material attached to the ring's outer rim. Underneath the magnetic ring and inside the magnetic cuff is a coil of wire through which electric current, representing the sounds from the distant telephone, flows. A thin diaphragm of magnetic material is suspended from the inside edges of the magnetic ring so it is positioned between the magnet and the coil. The magnetic field created by the magnet changes with the current in the coil and makes the diaphragm vibrate. The vibrating diaphragm creates sound waves that replicate the sounds that were transformed into electricity by the other person's transmitter.
3) ALERTER:
The alerter in a telephone is usually called the ringer, because for most of the telephone's history, a bell was used to indicate a call. The alerter responds only to a special frequency of electricity that is sent by the exchange in response to the request for that telephone number. Creating an electronic replacement for the bell that can provide a pleasing yet attention-getting sound at a reasonable cost was a surprisingly difficult task. For many people, the sound of a bell is still preferable to the sound of an electronic alerter. However, since a mechanical bell requires a certain amount of space in the telephone to be effective, smaller telephones mandate the use of electronic alerters.
4) DIAL:
The telephone dial has undergone major changes in its history. Two forms of dialing still exist within the telephone system: dial pulse from a rotary dial, and multifrequency tone, which is commonly called by its original trade name of Touch-Tone, from a push-button dial.
In a rotary dial, the numerals one to nine, followed by zero, are placed in a circle behind round holes in a movable plate. The user places a finger in the hole corresponding to the desired digit and rotates the movable plate clockwise until the user's finger hits the finger stop; then the user removes the finger. A spring mechanism causes the plate to return to its starting position, and, while the plate is turning, the mechanism opens an electrical switch the number of times equal to the dial digit. Zero receives ten switch openings since it is the last digit on the dial. The result is a number of "dial pulses" in the electrical current flowing between the telephone set and the exchange. Equipment at the exchange counts these pulses to determine the number being called.
The rotary dial has been used since the 1920s. But mechanical dials are expensive to repair and the rotary-dialing process itself is slow, especially if a long string of digits is dialed. The development of inexpensive and reliable amplification provided by the introduction of the transistor in the 1960s made practical the design of a dialing system based on the transmission of relatively low power tones instead of the higher-power dial pulses.
Today most telephones have push buttons instead of a rotary dial. Touch-Tone is an optional service, and telephone companies still maintain the ability to receive pulse dialing. Push-button telephones usually have a switch on the base that the customer can set to determine whether the telephone will send pulses or tones.
