A vacuum tube, also called a ‘valve’ in British English, is a device that controls electric current flow in a high vacuum between electrodes (conductors that emit or receive electrons). Tubes were used in many radios, television sets, and amplifiers until they were supplanted by lower cost transistors in the 1960s that performed the same function but used less electricity and were more durable.
In a vacuum tube, a cathode (an electrode that emits electrons) is heated, as in a light bulb, so it will emit electrons. This is called ‘thermionic emission.’ The electrons are accelerated from the cathode to the anode (an electrode that receives electrons) by the electric field in the tube. Vacuum tubes must be hot to work. Most are made of glass, thus are fragile and can break. Vacuum tubes were used in the first computers like the ENIAC, which were large and need much work to continue operating.
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The Daily Omnivore 