Technology & Medicine › Electric age › Radio
Radar
1935 (Watson-Watt; developed in several nations)· Invention or discovery· Communication & computing
Mostly agreedGood evidence; some details, dates or numbers uncertain.
Radar finds distant objects such as aircraft and ships by sending out radio pulses and timing the echoes that bounce back.
See it on the Technology & Medicine map →How it works
A transmitter fires short radio pulses; an antenna catches the faint echoes, and the delay gives the distance. In February 1935 Robert Watson-Watt and Arnold Wilkins showed that radio waves bouncing off a bomber could be detected. By 1939 Britain's Chain Home stations watched its coasts, helping win the 1940 Battle of Britain. Germany, the US, the USSR, Japan and others built radar in secret too. The British cavity magnetron (1940) made compact microwave radar possible.
Why it mattered
Radar shaped World War II's air and sea battles and now guides air traffic, weather forecasting and shipping. Wartime microwave work also produced the microwave oven.
Think of it like
Like shouting in a canyon and timing the echo to judge how far away the cliff is.
How we know
Watson-Watt's 1935 memo 'Detection and Location of Aircraft by Radio Methods', declassified wartime records, and parallel patents and lab files from several countries.
Still unknown or debated
Credit is shared: Germany's Christian Hülsmeyer patented a radio-echo ship detector in 1904, and several nations developed radar separately in the 1930s.
Connections (4)
▸Radio led to RadarWell documented
The connection
Radar is radio used as an echo-finder. Heinrich Hertz noticed in 1887–88 that radio waves bounce off metal, and in 1904 Christian Hülsmeyer patented a ship detector. In the 1930s stronger transmitters, sensitive receivers and precise pulse timing turned the idea into practical air defense.
When and how
Karlsruhe, 1887–88 (Hertz); Cologne, 1904 (Hülsmeyer); Britain, the US, Germany, the USSR, France and Japan in the 1930s.
How we know
Hülsmeyer's patent and Watson-Watt's 1935 memo survive; Watson-Watt was a radio researcher studying storms, and early radar used ordinary radio tubes and antennas.
Debates and open questions
Who invented radar has no single answer: many nations developed it in secret, so credit depends partly on which country's records you read.
▸The airplane (Wright brothers' first flight) created the air threat that spurred RadarWell documented
The connection
By the 1930s bombers could reach a city before anyone saw them coming; in 1932 Britain's Stanley Baldwin warned that 'the bomber will always get through'. The Air Ministry asked whether a radio 'death ray' could stop aircraft. Watson-Watt said no, but proposed detecting them instead. Radar was the answer.
When and how
Britain, 1934–35: the Air Ministry's Committee for the Scientific Survey of Air Defence (the Tizard Committee), with Watson-Watt and Arnold Wilkins.
How we know
Air Ministry files and Watson-Watt's February 1935 memo record both the 'death ray' question and the detection proposal.
Debates and open questions
Some historians say radar's role in the 1940 Battle of Britain is overstated compared with the ground-control system that turned its data into orders for fighters.
▸Radar spurred the semiconductor research behind the TransistorMostly agreed
The connection
Wartime microwave radar needed crystal detectors that could handle signals too fast for tubes. US and British labs learned to purify germanium and silicon to make them, building expertise in semiconductors, materials whose conductivity can be tuned. Bell Labs' transistor team built directly on that wartime crystal work.
When and how
MIT Radiation Laboratory, Purdue University, the University of Pennsylvania and Bell Labs, 1940–45; Russell Ohl found the silicon p–n junction at Bell Labs in 1940.
How sure are we?
Wartime research reports and Bell Labs histories show the transistor team used materials and methods from radar crystal work.
Debates and open questions
Purdue researchers saw related effects by 1947; why Bell Labs got to the transistor first is a classic case study in research management.
▸Radar shared pulse-echo principles with Medical ultrasoundMostly agreed
The connection
Radar sends out radio pulses and times the echoes to find aircraft. Ultrasound does the same with sound: a probe sends short, very high-pitched pulses into the body and times the echoes from each tissue boundary to locate it. Wartime radar electronics, plus sonar and metal-flaw detectors using the same pulse-echo idea, supplied the hardware.
When and how
Radar in Britain, the US, Germany and elsewhere in the 1930s–40s; Ian Donald, who learned about radar and sonar in the RAF during the war, began ultrasound work in Glasgow in the mid-1950s.
How sure are we?
Donald credited his wartime RAF exposure to radar and sonar, and early ultrasound machines used the same pulse timing and oscilloscope screens as radar.
Debates and open questions
Sonar, which uses sound underwater, is arguably a closer parent than radar; how to split credit among radar, sonar and industrial flaw detectors is a matter of emphasis.