The vacuum tube grew out of the light bulb and created electronics
Heat a filament inside an airless glass bulb and it sheds electrons. Add a positively charged plate and they flow one way only. That trick, first packaged as a practical device in 1904, underpinned radio, television, radar and the earliest computers, and it still survives inside microwave ovens and many guitar amplifiers.
Nineteenth-century experimenters, among them Thomas Edison and Nikola Tesla, had already played with evacuated glass tubes, though apart from light bulbs these stayed curiosities or laboratory tools. The first working vacuum tube, John Ambrose Fleming's diode of 1904, contained just a heated cathode and an anode. Current could pass only from one to the other, like water through a one-way valve, which is why Britons still call it a thermionic valve. Around 1913 H. J. Round introduced indirectly heated cathodes, a hidden heater warming an oxide-coated sleeve that emits electrons far more readily.
The real breakthrough was the grid. Slip a sparse mesh between cathode and anode and a small voltage on it can throttle a large current, so the tube amplifies as well as rectifies. More grids produced tetrodes, pentodes and beyond, some able to mix several signals. Engineers designed circuits from characteristic curves printed in tube manuals.
For the first half of the 20th century these glowing bottles made electronics a discipline. Earlier technologies such as spark transmitters and crystal detectors existed, but tubes made radio, sound recording, long-distance telephony and early digital computing practical. They had drawbacks: they wasted heat even when idle, could not be cooled by convection in their vacuum, and wore out as filaments burned through or cathode coatings were used up, so they plugged into sockets for easy replacement. Some carried a connection on top, called a top cap, to keep sensitive or high-voltage leads apart.
Semiconductors, invented in the 1940s, proved smaller, cooler, safer and more reliable, and from the mid-1960s transistors pushed tubes aside. The cathode-ray tube hung on in televisions and monitors until the early 21st century. Tubes remain in the magnetron of microwave ovens and some high-frequency amplifiers, and many audio fans, especially guitarists, still prize what they call a warmer sound.
Source: Vacuum tube
