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What is Vacuum Tube

WHAT IS VACUUM TUBE, HISTORY OF VACUUM TUBE
A vacuum tube is, reasonable enough, a sealed glass tube containing a vacuum in which are present several electronic elements: the cathode, anode, grid, and filament. When the cathode and anode are heated by the filament, and a voltage is applied across them, current flows between the cathode and anode. If a grid is inserted between them, the flow can be controlled by changing the grid between a positive and negative voltage. 
The grid voltage can be quite small, and the plate voltages can be quite high, thus providing an amplifying capability. More importantly for computers, switching the grid voltage causes the tube to act as a switch with respect to the plates. Thus, we have an electronically controlled switch that is much faster than a relay.
A type of vacuum tube also served as a popular storage mechanism, the Cathode Ray Tube (CRT). Other memory devices used during the period include mercury or glass delay lines, and magnetic core memory. Vacuum tubes, however, are large, require alot of power, and produce a lot of waste heat. in fact for one rather large vacuum tube machine, it was once estimated that if its four turbine-powered air conditioners were to fail, the heat buildup in 15 minutes would be sufficient to melt the concrete and steel building containing if. it has also been  estimated that if a modern computer were built with vacuum tubes, it  would be the size of the Empire State building. 

Some Machines Name which used Vacuum Tube:
  1. Atanasoff's Machine (1940)
  2. COLOSSUS (1943)
  3. ENIAC (1946)
  4. MADM (1948)
  5. EDVAC and EDSAC (1949)
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