Technology & Medicine › Electric age › Radio
Vacuum tube
1904 (Fleming diode); 1906 (De Forest triode)· Invention or discovery· Communication & computing
Well documentedSolid records, physical evidence or scientific consensus.
A sealed glass tube in which a heated filament sends electrons across a vacuum, used to detect, amplify and switch electric signals.
See it on the Technology & Medicine map →How it works
John Ambrose Fleming, a consultant to Marconi, patented the diode in 1904: a heated filament and a metal plate let current flow only one way, turning radio waves into a detectable signal. In 1906 Lee de Forest added a grid between them, creating the triode, or 'Audion'. A small voltage on the grid controls a large current, so the tube can amplify. By 1913–15 improved high-vacuum tubes boosted long-distance phone lines and powered radio transmitters.
Why it mattered
The vacuum tube was the first electronic amplifier and switch, making possible radio broadcasting, coast-to-coast calls, television, radar and the first electronic computers.
Think of it like
Like a valve on a water main: a small turn of the handle controls a big flow. The British still call tubes 'valves'.
How we know
Fleming's 1904 British patent; de Forest's patents and the long court fights over them; surviving tubes and lab records from AT&T and General Electric, where high-vacuum tubes were perfected around 1913.
Still unknown or debated
De Forest seems not to have understood how his own triode worked at first; how much credit goes to him versus Edwin Armstrong and Irving Langmuir is still argued.
Connections (4)
▸Incandescent light bulb led, via the Edison effect, to the Vacuum tubeWell documented
The connection
Around 1880–83 Edison noticed his bulbs blackening inside and sealed in a metal plate to investigate; current flowed from the hot filament to the plate, but only one way. He patented it but found no real use. Fleming, a former adviser to Edison's London company, used this 'Edison effect' in 1904 to build the diode, the first vacuum tube.
When and how
Menlo Park, New Jersey, 1880–83 (Edison); London, 1904 (Fleming, then working for the Marconi company).
How we know
Edison's 1884 patent for an electrical indicator uses the effect; Fleming published studies of it in the 1880s and 1890s, and his 1904 diode patent built directly on it.
Debates and open questions
Frederick Guthrie in Britain saw a related effect in 1873, so the name arguably over-credits Edison; nobody understood it until the electron's discovery in 1897.
▸Vacuum tube amplified RadioWell documented
The connection
Early radio receivers could barely detect weak signals. The triode amplified them, and tubes wired to feed back on themselves could also generate steady waves to carry voice and music. Edwin Armstrong's feedback circuit (1912–14) and later designs made broadcast transmitters and loudspeaker sets possible. Tubes ran radios until transistors took over in the 1950s–60s.
When and how
Fleming in London, 1904; de Forest in New York, 1906; Edwin Armstrong at Columbia University, 1912; KDKA's tube transmitter in Pittsburgh, 1920.
How we know
Patents, engineering papers and surviving tube radios document the link; the feedback-circuit patent fight between Armstrong and de Forest reached the US Supreme Court in 1934.
Debates and open questions
The courts gave the feedback circuit to de Forest, but most engineers credited Armstrong, and the argument continues among historians.
▸Vacuum tube was the switch inside the Electronic computer (Z3, Colossus, ENIAC)Well documented
The connection
A vacuum tube can switch current on and off thousands of times faster than a mechanical relay, with no moving parts. Colossus used 1,600–2,400 tubes and ENIAC about 18,000 as on-off switches representing numbers. Tubes were power-hungry and failed often, so engineers ran them gently and replaced them constantly.
When and how
Iowa State College (Atanasoff–Berry Computer, 1942); Bletchley Park, England (Colossus, 1944); University of Pennsylvania, Philadelphia (ENIAC, 1945).
How we know
Machine specifications, maintenance logs and photographs survive; Tommy Flowers drew on his experience with tubes in the Post Office's telephone exchanges.
Debates and open questions
Experts doubted thousands of tubes could work reliably together; Flowers proved them wrong by never switching them off. Whether Colossus or ENIAC counts as the first electronic computer is still argued.
▸Vacuum tube was replaced by the TransistorWell documented
The connection
The transistor did a vacuum tube's jobs, amplifying and switching, with a speck of crystal instead of a heated filament in a glass bulb. It was smaller, used far less power, gave off little heat and lasted for years. Through the 1950s and 1960s transistors replaced tubes in radios, computers and telephone systems.
When and how
Bell Labs, New Jersey, 1947; transistor radios from 1954; transistor computers from 1953–55; most consumer tubes gone by the 1970s.
How we know
Industry production records and product histories trace the switch; Bell Labs said from the start that replacing tubes was the goal.
Debates and open questions
Tubes never fully vanished: high-power transmitters, microwave ovens' magnetrons and many guitar amplifiers still use them, and audiophiles argue over their warmer sound.