Coolidge tube, used by Dr. Burkitt, made by General Electric, USA, c.1920

American physicist William Coolidge (1832-1919) patented his ‘hot cathode’ tube in 1913. The cathode was a heated, electron-emitting tungsten filament. It produced a high output of X-rays that did not fluctuate and were easily adjusted. This eliminated many problems of the older so-called ‘gas tubes’. Combined with more reliable power supplies, it allowed X-ray techniques to become standardised and reproducible. The fins at the end of the tube disperse the heat generated in the tungsten anode which the larger currents use.