Battles & Wars › World War II › Radar (and Britain's Chain Home)
Cavity magnetron and the Tizard Mission
invented February 1940; shared with the US September 1940· Invention or discovery· Technology, codes & intelligence
Well documentedSolid records, physical evidence or scientific consensus.
A small device that generated powerful short-wave microwaves, making compact, precise radar possible; Britain gave it to America in 1940.
See it on the Battles & Wars map →What happened
On February 21, 1940, John Randall and Harry Boot at the University of Birmingham ran a copper block drilled with cavities, in which electrons spun by a magnet produced strong microwaves about 10 cm long. Short waves meant small antennas and sharp images, so radars could fit into aircraft and small ships. In September 1940 the Tizard Mission, led by scientist Henry Tizard, carried a magnetron and other secrets to the still-neutral US, seeking American factory capacity in return.
Why it matters
It gave the Allies microwave radar well ahead of Germany, helping decide the U-boat war and night air battles; it is also the heart of the microwave oven.
Think of it like
Like blowing across a bottle's mouth to make a tone, except electrons whirling past copper cavities make radio waves.
How we know
Randall and Boot's lab records, the Tizard Mission papers, and surviving early magnetrons. The official US history called it 'the most valuable cargo ever brought to our shores'.
Still debated
Soviet and Japanese researchers built similar devices in the 1930s; how independent each line of work was is still discussed.
Connections (4)
▸Cavity magnetron and the Tizard Mission let patrol aircraft find U-boats in the Battle of the AtlanticWell documented
The connection
Earlier long-wave airborne radar gave weak returns and could be detected by U-boat receivers. Centimetric radar based on the magnetron, such as Britain's ASV Mark III from early 1943, could spot a surfaced submarine at night, and Germany had no warning receiver for it for months. Paired with the Leigh searchlight, aircraft caught U-boats surfacing to recharge batteries.
When and how
Bay of Biscay and North Atlantic, 1943; RAF Coastal Command and US aircraft, plus escort ships fitted with Type 271 centimetric radar from 1941.
How we know
Admiralty and Coastal Command records, and German U-boat command diaries complaining of surprise night air attacks.
Debates and open questions
Dönitz blamed radar for losses partly caused by codebreaking, which German inquiries repeatedly ruled out; separating the two causes remains hard.
▸Cavity magnetron and the Tizard Mission guided the night raids of the Strategic bomberWell documented
The connection
The magnetron powered H2S, a downward-looking radar that showed bomber navigators a rough map of coastlines, rivers and towns through cloud and darkness. RAF Bomber Command first used it in January 1943; US bombers used a version called H2X. Germany recovered a magnetron from a bomber shot down near Rotterdam in February 1943.
When and how
Over Germany, January 1943 onward; RAF Bomber Command's Pathfinder Force, and the US Eighth Air Force with H2X from late 1943.
How we know
RAF operational records, the Telecommunications Research Establishment's papers, and German reports on the captured 'Rotterdam device'.
Debates and open questions
H2S was poor at finding small targets; how much it improved bombing results, versus simply allowing raids in bad weather, is debated.
▸MIT Radiation Laboratory turned into working radars the Cavity magnetron and the Tizard MissionWell documented
The connection
The Tizard Mission's magnetron worked, but turning it into radar sets needed receivers, antennas, displays and factories. The Rad Lab, founded weeks after the mission arrived, had a working microwave radar by early 1941 and went on to design many of the microwave radars the Allies used.
When and how
Cambridge, Massachusetts, from late 1940; physicists such as Lee DuBridge, Luis Alvarez and I. I. Rabi, advised by Britain's Edward 'Taffy' Bowen.
How we know
The Rad Lab's own reports and technical series, and British and US records of the Tizard Mission's handover.
Debates and open questions
How much Britain gained in return, beyond American factory capacity, is debated; some British scientists felt the US later took the credit.
▸Raytheon (radar magnetrons to the Patriot missile) mass-produced most Allied units of the Cavity magnetron and the Tizard MissionMostly agreed
The connection
Britain's Tizard Mission brought the cavity magnetron to the US in September 1940. Each one had to be machined from solid copper. Raytheon's Percy Spencer devised a way to stamp out thin copper layers and join them, raising output from dozens a day to about 2,600 a day. Raytheon is credited with about 80% of Allied wartime magnetrons.
When and how
Waltham and Newton, Massachusetts, 1941–45; Percy Spencer and Raytheon's workforce.
How sure are we?
Spencer's patents and company production records; the figures come mostly from Raytheon's own histories.
Debates and open questions
Sources disagree on the starting rate, from about 17 to 100 a day, and on what the 80% counts: all magnetrons, or only radar ones.