 Oliphant 's group at Birmingham worked on developing two promising devices , the klystron and the magnetron . Working with James Sayers , Oliphant managed to produce an improved version of the klystron capable of generating 400W . Meanwhile , two more members of his Birmingham team , John Randall and Harry Boot , worked on a radical new design , a cavity magnetron . By February 1940 , they had an output of 400W with a wavelength of 9 @.@ 8 centimetres ( 3 @.@ 9 in ) , just the kind of short wavelengths needed for good airborne radars . The magnetron 's power was soon increased a hundred @-@ fold , and Birmingham concentrated on magnetron development . The first operational magnetrons were delivered in August 1941 . This invention was one of the key scientific breakthroughs during the war and played a major part in defeating the German U @-@ boats , intercepting enemy bombers , and in directing Allied bombers . 
