Early magnetrons (negative resistance magnetrons and cyclotrons) were inefficient and had no practical significance. The first multi-cavity magnetron was made by Soviet engineers Н.Ф. Alexei Yavi and Д.Е. Ma Liao Luofu made between 1936 and 1937. In 1939, the British physicist H.A.H. Bout and J.T. Randall also made a multi-cavity magnetron. In the Second World War, multi-cavity magnetrons were widely used in military radar transmitters, played a significant role. By 1945, its operating frequency has reached 30 GHz. Generally known as the magnetron, that is, multi-cavity magnetron.