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The development of vacuum electronics has started in the second half of the 19th century when the first light bulbs, discharge lamps and vacuum tubes were created. During his light bulb research, Thomas Edison found out that when an electrode is placed inside a light bulb and a voltage is connected between it and the filament, small current flows through the vacuum between the electrode and the filament, but only in one direction. He did not manage to further examine this phenomenon. Other scientists later discovered that the current flows because of free electrons that are emitted by the white hot filament. Lee de Forest, an American inventor, tried to put another electrode (grid) between the filament (cathode) and the other electrode (anode) in 1906 and found out that the current flow between the catode and the anode can be controlled by grid voltage variation. This is how the first electron tube that could amplify an electric signal was created. The era of electronics could begin. Later, tubes with more grids (up to seven), TV picture tubes, transmitting tubes and various special designs for military or scientific devices were invented. The vacuum technology has been almost completely superseded by semiconductors, which are cheaper and much better suited for miniaturisation in case that their manufacture is well designed. After the classic glass CRT TV receivers and monitors were outplaced by LCD technology, the only commonly used vacuum tube in households is the magnetron in microwave ovens. In lighting technology, various light bulbs and gas discharge lamps are slowly superseded by LED diodes. Vacuum tubes are still used in case that high power at high frequency is needed.
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