Telephone

Telephone An instruments that transmits the spoken word by means of electrical impulses through wires. Alexander Graham Bell invented the telephone in the united states in 1876. When you speak into the transmitter of the telephone, the sound waves vibrate a diaphragm.

This presses against grains of carbon, whose electrical resistance varies according to the pressure upon it. An electric current passes through the carbon grains, and varies in strength according to the pressure exerted upon them by the vibrating diaphragm.

The Variable current signals thus set up travel to the receiver (earpiece) of the telephone of the person you are calling. There they pass through an electromagnet over which is located another diaphragm. The diaphragm vibrates in sympathy with the signals and reproduces the sounds that originally went into the transmitter.

And Now - Digital telephone generation, called Handphone.

Loudspeaker

Loudspeaker A device that converts electrical signals into sound waves. It works in the opposite way to a microphone. The commonest type is the moving-coil speaker, illustrated here. A coil of wire is attached to the apex of a large paper cone and is located between the poles of a permanent magnet. When an electrical current passes through the coil, it sets up a magnetic field.

This field interacts with that of the magnet and causes the coil to be attracted or repelled. This vibrates the cone, which causes sound to be emitted. In HI-FI equipment, saveral separate speakers are included in the speaker unit, each sensitive to a particular frequency range. The high-frequency one is called a tweeter; the low-frequency one, a woofer.

Tape Recording

Recording sound by means of magnetic tape. Valdemar Poulsen in Denmark discovered the basic principles in the 1890s, recording on megnetized wire. Tape recordings are done on plastic tape coated with magnetic iron oxide or chromium oxide partocles. In recording, sounds are converted by a microphone into variable electrical signals. These are then fed through the coils of an electromagnet and cause its magnetism to fluctuate in sympathy.

This fluctuating magnetism is recorded as an invisible pattern on magnetic tape moving past the electromagnet and induces in it variable electrical signals. These mimic the original signals produced by the microphone and are fed through a loudspeaker to reproduce the original sounds. In video tape recording, signals from a television camera are recorded on magnetic tape, and are played back through a television receiver to reproduce pictures..........

And Now ...this one only old Technology

Television

The Transmission of moving pictures by radio waves (in broadcasting) or by cable (in closed-circuit television). John Logie Baird in Britain developed the first practical television system in 1925 using a mechanical scanning process. But this process was soon replaced by the modern method of electronic scanning, pioneered by Vladimir Zworykin in the United States.

In a simple black and white television camera, a lens focuses an image on a light-sensitive plate in the camera tube. Electrons are emitted from the various parts of the plate according to the amount of light reaching them. They travel to a target plate and make it electrically charged. The pattern of charges on the target is an electrical representation of the optical image formed on the light-sensitive plate by the lens.

The Target is then scanned rhythmically from left to right and line by line, by a beam of electrons from an electron gun. The electron beam interacts with the charges on the target to produce an electric current, which varies according to the amount of charge there - that is, on the brightness of each part of the picture. This variable current comprises the video signals, which are then transmitted, together with a synchronizing signal on a radio carrier wave.

A Television receiver picks up the transmitted picture signals, via an aerial, and first separates the video signals from the carrier wave. The signals are then fed to a cathode-ray tube, where they are made to vary the strength of an electron beam. The beam strikes the fluorescent viewing screen, causing it to glow more or less according to the strength of the beam. At the same time the beam is made to scan the screen side to side and line by line in the same way as the beam did in the camera tube. (625 line are common in Britain.) In this way a pattern of light and dark is built up on the screen, which reproduces the original scene viewed by the television camera.

In colour television, the coloured light entering the camera is split up by filters into different combinations of the three primary colours red, blue and green. Each colour is then fed to a separate camera tube, which produces its own signals. These signals are then transmitted and picked up by the receiver. There they are fed to three separate electron guns in the picture tube. Beams of electrons representing red, blue and green light are then directed onto phosphors on the screen that glow respectively red, blue, and green. The phosphors are so close together that their colours merge to reproduce the original colour.

Thermionic Valves

Electron tubes in which electrons are emitted from a heated electrode (cathode) . The British physicist John Ambrose Fleming developed the first valve in 1904 for use in radio receivers. It was a two electrode valve or diode, containing a cathode and an anode in an evacuated glass bulb. Two years later the American Lee De Forest produced a triode, which had a third electrode (grid) between the cathode and the anode. In the triode, a small change of voltage on the grid has a great effect on the electron flow, or current passing between cathode and anode. in this way signals can be amplified. Thermionic valves with further grids are available for different uses. For many applications these days, however, thermionic valves have been replaced by semiconductor devices, such as Transistors