Watson-Watt's radar equipment, 1935.
© Science Museum / Science & Society Picture Library
Equipment used by Sir Robert Watson-Watt (1892-1973) for the detection of radio echoes from aircraft in February 1935. Watson-Watt had developed a system for locating thunderstorms from their radio emisions, which he then went on to develop into a means of locating aircraft. In 1939, the invention of the magnetron, a means of generating very short radio waves (microwaves), made radar posible in practice. Radar (radio detection and ranging) works by bouncing radio waves off distant objects. By measuring the total time taken for the beam to travel to the target and back, it is posible to calculate how far away it is. The bearing of the target is obviously the direction from which the reflected beam is received.