Tento článok bol vytlačený zo stránky https://referaty.centrum.sk


Alexander Graham Bell biography

Bell, Alexander Graham (1847-1922), American inventor and teacher of the deaf, most
famous for his invention of the telephone.

Bell was born on March 3, 1847, in Edinburgh, Scotland, and educated at the universities of Edinburgh and London. He immigrated to Canada in 1870 and to the United States in 1871. In the United States he began teaching deaf-mutes, publicizing the system called visible speech. The system, which was developed by his father, the Scottish educator Alexander Melville Bell, shows how the lips, tongue, and throat are used in the articulation of sound. In 1872 Bell founded a school to train teachers of the deaf in Boston, Massachusetts. The school subsequently became part of Boston University, where Bell was appointed professor of vocal physiology. He became a naturalized U.S. citizen in 1882.

Since the age of 18, Bell had been working on the idea of transmitting speech. In 1874, while working on a multiple telegraph, he developed the basic ideas for the telephone. His experiments with his assistant Thomas Watson finally proved successful on March 10, 1876, when the first complete sentence was transmitted: "Watson, come here; I want you." Subsequent demonstrations, particularly one at the 1876 Centennial Exposition in Philadelphia, Pennsylvania, introduced the telephone to the world and led to the organization of the Bell Telephone Company in 1877.

In 1880 France bestowed on Bell the Volta Prize, worth 50,000 francs, for his invention. With this money he founded the Volta Laboratory in Washington, D.C., where, in that same year, he and his associates invented the photophone, which transmits speech by light rays. Other inventions include the audiometer, used to measure acuity in hearing; the induction balance, used to locate metal objects in human bodies; and the first wax recording cylinder, introduced in 1886. The cylinder, together with the flat wax disc, formed the basis of the modern phonograph.

Bell was one of the cofounders of the National Geographic Society, and he served as its president from 1896 to 1904. He also helped to establish the journal Science by financing it from 1883-1894.

After 1895 Bell's interest turned mostly to aeronautics. Many of his inventions in this area were first tested near his summer home at Baddeck on Cape Breton Island in Nova Scotia, Canada. His study of flight began with the construction of large kites, and in 1907 he devised a kite capable of carrying a person.

With a group of associates, including the American inventor and aviator Glenn Hammond Curtiss, Bell developed the aileron, a movable section of an airplane wing that controls roll. They also developed the tricycle landing gear, which first permitted takeoff and landing on a flying field. Applying the principles of aeronautics to marine propulsion, his group started work on hydrofoil boats, which travel above the water at high speeds. His final full-sized "hydrodrome," developed in 1917, reached speeds in excess of 113 km/h (70 mph) and for many years was the fastest boat in the world.

Bell's continuing studies on the causes and heredity of deafness led to experiments in eugenics, including sheep breeding, and to his book Duration of Life and Conditions Associated with Longevity (1918). He died on August 2, 1922, at Baddeck, where a museum containing many of his original inventions is maintained by the Canadian government.

Telegraph, system of communication employing electrical apparatus to transmit and receive signals in accordance with a code of electrical pulses. Originally the term telegraphy referred to any form of communication over long distances in which messages were transmitted by signs or sounds.

II. The Morse Telegraph

The first electrical instruments for telegraphic transmission were invented in the United States by the American inventor Samuel F. B. Morse in 1837 and in Britain the same year by the British physicist Sir Charles Wheatstone in collaboration with the British engineer Sir William F. Cooke. Morse used a simple code in which messages were transmitted by electric pulses passing over a single wire (see Morse Code, International). Morse's apparatus, which sent the first public telegram in 1844, resembled a simple electric switch. It allowed current to pass for a prescribed length of time and then shut it off, all at the pressure of a finger. The original Morse receiver had an electromagnetically controlled pencil that made marks on paper tape moving over a clockwork-operated cylinder. The marks varied with the duration of the electric current passing through the wires of the electric magnet and took the written form of dots and dashes.

While experimenting with his instrument, Morse found that signals could be transmitted successfully for only about 32 km (20 mi). Beyond that distance the signals grew too weak to be recorded. Morse and his associates therefore developed a relay apparatus that could be attached to the telegraph line 32 km from the signal station to repeat signals automatically and send them an additional 32 km. The relay consisted of a switch operated by an electromagnet.

An impulse entering the coil of the magnet caused an armature to rotate and close an independent circuit actuated by a battery. This action sent a fresh pulse of current into the line, and this pulse in turn activated successive relays until the receiver was reached. A few years after Morse developed his receiving instrument and demonstrated it successfully, telegraph operators discovered that it was possible to distinguish dots and dashes by sound alone, and the Morse recording apparatus was therefore discarded. The other fundamental principles of the Morse system continued in use in wire-telegraph circuits, however.

Because telegraphy was too expensive for widespread use, several means of sending several messages simultaneously over a single line were developed. In duplex telegraphy, the earliest advance of this kind, one message can be transmitted simultaneously in each direction between two stations. In quadruplex telegraphy, invented in 1874 by the American engineer Thomas Edison, two messages were transmitted in each direction simultaneously. In 1915 multiplex telegraphy came into use, permitting the transmission of eight or more messages simultaneously. Because of this and the development of teleprinting machines during the mid-1920s, the Morse manual telegraph system of code and key was gradually discontinued for commercial use and replaced by automatic wire and wireless radio-wave methods of transmission.

III. Automatic Telegraph Systems

There are two basic systems of modern telegraphic communication: the teleprinting system (teletype), which is still in use, and the facsimile reproduction system, which became obsolete in the 1980s.

A. Teleprinting

In teleprinting, the message is received in the form of typed words on a paper form. In this system each letter of the alphabet is represented by one of 31 combinations of five equal-interval electronic impulses, with the sequence of used and unused intervals determining the letter. The start-stop printing code uses seven pulses for each character, the first pulse indicating the beginning and the seventh pulse the end of the letter.

The transmitter or teletypewriter consists of a typewriterlike keyboard and may or may not record the message on tape before it is transmitted. The receiver is basically like a typewriter without a keyboard that prints the message on a tape or a paper form. Most machines in the start-stop system are both transmitters and receivers. News organizations were among the major users of the teletype and similar communications systems.

By the early 1990s, however, press associations and broadcast media transmitted both text and pictures electronically via satellite.

B. Facsimile Reproduction

Facsimile telegraph systems, which send and receive images and text, have been rendered largely obsolete by facsimile transmission, commonly referred to as fax.

IV. Telegraph Carrier Media

The electrical impulses that make up telegraph messages may be carried through wire circuits or may be broadcast as radio waves.

When Morse invented the telegraph, the only way that a message could be carried from one point to another was by wires strung directly from the transmitting device to the receiver, regardless of the distance. The wire could carry only one message at a time, and reamplification and signal correction devices had to be set up at regular points along the line. By utilizing carrier currents, which are alternating currents of a number of different frequencies, a single pair of wires can simultaneously transmit hundreds of messages, for each frequency represents a transmission channel (see Carrier Wave; Frequency). The various channels are combined at the sending station into the carrier current transmitted by the telegraph wires. At the receiving end the carrier current is passed through electrical filters, each of which transmits only a particular frequency to an appropriate receiving device. Thus, a great number of individual channels may be obtained with only one electrical circuit.

V. Microwave Transmission

The use of microwave radio transmission for long-distance telegraphic communication all over the world grew to be of major importance after World War II ended in 1945 (see Radar). The first commercial microwave radio link in telegraphy began operation between Philadelphia and New York City in 1947. It was followed in 1948 by a three-way network linking New York City, Washington, D.C., and Pittsburgh. The system then spread rapidly across the United States through the use of microwave relay antenna towers.

Microwave telegraphy is capable of carrying vocal, printed, graphic, photographic, and video communication almost instantaneously and in large quantities. It operates in the 4000-megahertz range of the commercial communications band. In this range, 40 voice bands are available in either direction, providing about 800 telegraph channels. The radio signals originating at the broadcast source are relayed to their destination by a series of parabolic reflector antennas mounted at the top of tall masts.

In order to overcome weakening of the signal by distance and the curvature of the earth, these microwave relay antennas are placed at line-of-sight intervals about 48 km (about 30 mi) apart. This microwave transmission service was established in the U.S. by the Western Union Telegraph Company. For intercontinental communication, artificial geosynchronous satellites are used as relay antennas for voice, data, graphic, and video signals between ground-based stations (see Communications Satellite; Space Exploration).

I. 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 in an emergency (as in the 911 service in the United States). 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 1998 there were 661 main telephone lines per 1,000 people in the United States and 634 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.

II. Parts of a Telephone

A basic telephone set contains a transmitter that transfers the caller's voice; a receiver that amplifies sound from an incoming call; a rotary or push-button dial; a ringer or alerter; and a small assembly of electrical parts, called the antisidetone network, that keeps the caller's voice from sounding too loud through the receiver. If it is a two-piece telephone set, the transmitter and receiver are mounted in the handset, the ringer is typically in the base, and the dial may be in either the base or handset. The handset cord connects the base to the handset, and the line cord connects the telephone to the telephone line.

More sophisticated telephones may vary from this pattern.

A speakerphone has a microphone and speaker in the base in addition to the transmitter and receiver in the handset. Speakerphones allow callers' hands to be free, and allow more than two people to listen and speak during a call. In a cordless phone, the handset cord is replaced by a radio link between the handset and base, but a line cord is still used. This allows a caller to move about in a limited area while on the telephone. A cellular phone has extremely miniaturized components that make it possible to combine the base and handset into one handheld unit. No line or handset cords are needed with a cellular phone. A cellular phone permits more mobility than a cordless phone.

A. 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.

B. 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.

C. 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.

D. 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.

E. Business Telephones

A large business will usually have its own switching machine called a Private Branch Exchange (PBX), with hundreds or possibly thousands of lines, all of which can be reached by dialing one number. The extension telephones connected to the large business's PBX are often identical to the simple single-line instruments used in residences. The telephones used by small businesses, which do not have their own PBX, must incorporate the capability of accessing several telephone lines and are called multiline sets. The small-business environment usually requires the capability of transferring calls from one set to another as well as intercom calls, which allow one employee to call another without using an outside telephone line.

F. Cellular Telephones

A cellular telephone is designed to give the user maximum freedom of movement while using a telephone. A cellular telephone uses radio signals to communicate between the set and an antenna. The served area is divided into cells something like a honeycomb, and an antenna is placed within each cell and connected by telephone lines to one exchange devoted to cellular-telephone calls. This exchange connects cellular telephones to one another or transfers the call to a regular exchange if the call is between a cellular telephone and a noncellular telephone. The special cellular exchange, through computer control, selects the antenna closest to the telephone when service is requested. As the telephone roams, the exchange automatically determines when to change the serving cell based on the power of the radio signal received simultaneously at adjacent sites. This change occurs without interrupting conversation. Practical power considerations limit the distance between the telephone and the nearest cellular antenna, and since cellular phones use radio signals, it is very easy for unauthorized people to access communications carried out over cellular phones. Currently, digital cellular phones are gaining in popularity because the radio signals are harder to intercept and decode.

Here are 149,084,370 telephone lines in the world, with thousands more being added every day. It's a lot to keep track of...concentration is essential.

Koniec vytlačenej stránky z https://referaty.centrum.sk