A D V E N T U R E S   in   C Y B E R S O U N D

The working principles of electronics can be demonstrated by tracing the history of radio tubes and photoelectric cells.

The history began in 1883, when Thomas Edison found that the heated filament in his incandescent lamp gave off material that blackened the inside of the bulb. This was called the Edison effect, and it led to the development of the modern radio tube. In the Edison effect, also called thermionic emission, heat supplies some electrons in the filament with at least the minimal energy to overcome the attractive forces holding them in the structure of the metal. This discharge of electrons is widely used as a source of electrons in conventional electron tubes, for example, in television picture tubes.

In 1887 Heinrich Hertz, while trying to prove the existence of radio waves, discovered the photoelectric effect. If polished metal is given a negative charge and then is flooded with ultraviolet radiation, it steadily loses the charge. Some chemical elements such as cesium and selenium are sensitive to visible light. This discovery led to photoelectric cells.

. The development of the radio tube began in 1904, when John A. Fleming of England produced the Fleming valve, which today is called a diode, meaning "two electrodes." He started by heating a filament (also called a cathode) in a vacuum tube with "A-circuit current." The heat drove electrons out of the filament and into surrounding space. If nothing more happened, the first electrons to escape would soon have formed a negative space charge that would have kept others from being driven out because like charges repel.

Fleming avoided this by placing a plate in the tube and connecting the plate and filament through an outside B circuit. The electrons driven from the filament then crossed the tube to the plate and followed the circuit back to the filament. Fleming next placed a battery in the B circuit. The battery was used to supply electrons--that is, negative charges--to the filament, or cathode, and draw them from the plate, or anode, leaving a positive charge. Electrical heating drove electrons steadily from the filament and sent a strong current through the B, or plate, circuit.

The strength of the current depends partly upon the heat and partly upon the voltage from the battery. This device could be used as a radio detector. The changing voltages created by radio signals in an antenna circuit are placed on the filament and plate. The changes produce corresponding changes in the strength of the plate current, which is used to reproduce the signal in the receiving apparatus.

In 1906 the American inventor Lee De Forest transformed the diode into a device that he called an audion, the modern name of which is triode. He did this by inserting a grid of fine wire mesh between the filament and the plate. If variable voltages from an antenna circuit are placed on the filament and the grid, they cause variations in the flow of electrons to the plate. Moreover, the variations in current are much stronger than those caused by the voltage of the incoming signal acting alone. Thus the triode amplifies, or strengthens, the signal.

Because the tube uses free electrons only and has no mechanical moving parts, it responds within a few microseconds, or millionths of a second, to any change placed upon it. It can be made sensitive to changes of less than a millionth of a volt. Resulting changes in the plate current can be amplified by passing the signal through more tubes. The vacuum tube became the basis of radio, television, and computers, the latter first developed at the end of World War II in 1944 and 1945.

The invention of the transistor in 1948 initiated a radical reduction in the size of electronic circuits and in their power requirements. The later development of the integrated circuit set into motion the continuing miniaturisation of all electronic devices, which has at the same time greatly increased their speed and computing power.

Source: Lerner, Eric, J., 1992, Compton's Encyclopaedia

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