Robert Watson-Watt is given the credit for inventing the radar. In fact, this credit should go to the German engineer Christian Hulsmeyer who in 1904, using patented an early warning system for shipping. He, in turn, used a discovery by Heinrich Hertz who had discovered in 1888 that radio waves could be bounced off objects.
In 1935, Robert Watson-Watt - a Scottish physicist - was asked by the Air Ministry to investigate the possibility of creating a "death-ray" weapon using radio waves. Watson-Watt was working at the National Physical Laboratory in Slough.
Watson-Watt did not create a "death-ray" weapon but he did find that his radio transmitters could create an echo from an aeroplane that was over 200 miles away. This information would give the Royal Air Force an early warning of an attack by enemy fighters. By the time an enemy force was nearing our coastline, our fighters would be airborne and ready to fight. The enemy would have lost the element of surprise. This invention by Watson-Watt was vital to the RAF during the Battle of Britain in 1940.
In 1940, aided by John Randall and Henry Boot from Birmingham University, Robert Watson-Watt invented the cavity magnetron. This produced a compact source of short-wave radio waves and allowed Fighter Command of the RAF to detect incoming enemy planes from a much greater distance thus giving our pilots more time to organise themselves.
Those experts who operated the magnetron also found it had another use - it could heat up water. Today, magnetrons are used as the source of heat in microwave ovens.