Vacuum Tube
A vacuum tube is an electronic component in which electrons flow through an evacuated glass or metal envelope between a heated cathode and a positively charged anode, with the flow optionally controlled by one or more grids in between. It was the first device able to amplify electrical signals and to rectify and switch them without moving parts. For roughly half a century it was the active component of radio, telephony, television, radar and early computers, until the transistor, announced in 1948, took over its role in nearly all of those jobs.
The two inventions that defined the tube came a few years apart. John Ambrose Fleming, a British engineer, patented in 1904 a two-electrode device that he called a valve because it let current pass in only one direction, a use immediately valuable for detecting radio signals. In 1906 the American inventor Lee de Forest added a third electrode, a grid between cathode and anode, and called the result the audion. The grid made all the difference: a small voltage on it controlled a much larger current to the anode, so the triode could amplify. Later improvements, including better vacuums that were developed in the 1910s, made the triode reliable enough for the telephone network and radio broadcasting.
The structure is simple, and each part has a function. A heater warms the cathode until it emits electrons by thermionic emission, creating a cloud near its surface. The anode, also called the plate, is held at a positive voltage and collects them. Because there is almost no gas, the electrons travel without colliding with air molecules. The grid is a wire mesh close to the cathode; making it slightly negative repels electrons and reduces the plate current, while making it less negative lets more through. Tubes with more grids, such as tetrodes and pentodes, improved gain and reduced unwanted effects, and the same envelope idea produced cathode-ray tubes, X-ray tubes and microwave generators.
Tubes limited computers because they ran hot and wore out. The cathode must be kept glowing, so each tube consumed heater power whether or not it was doing work, and the heat had to be carried away. Heaters burn out, cathode coatings degrade, and glass can crack or leak, so a machine with thousands of tubes had a failure somewhere often enough to dominate its running. ENIAC, completed in 1945, contained about 17,468 vacuum tubes, filled a large room and drew power on the order of a hundred kilowatts. Designers kept tubes on at all times and used conservative operating conditions to reduce failures, because power-on and power-off cycles themselves stressed the filaments.
The usual misconception is that vacuum tubes disappeared when the transistor arrived. Transistors replaced them in computers, small radios and most signal processing within a couple of decades, but tubes continue in niches where their properties win: high-power radio and broadcast transmitters, microwave generators such as magnetrons in ovens and radar, traveling-wave tubes in satellites, X-ray tubes, and the guitar and audio amplifiers that musicians prefer for their distortion. Their role as a concept lives on: every transistor is a controlled-flow device that does the grid's job, a small input regulating a much larger output, only in solid state, with no heater and no evacuated glass.
Read the vacuum tube as the first amplifier, an evacuated glass bottle in which a hot cathode, a plate and a control grid let a small voltage steer a large current, powerful enough to create electronics and hungry and fragile enough that the transistor was invented to replace it.