In 1913, physicist William Coolidge of General Electric (Schenectady, USA) published his discovery of the X-ray tube with a filament for the heating of the cathode. This is a vacuum tube and, with the filament current, the intensity of the radiation can be well adjusted. The radiation characteristics are therefore more easily held constant than with the old Crookes gas tubes.
From the summer of 1915, Dr. Gaarenstroom in the Netherlands Cancer Institute uses a Coolidge type X-ray tube alongside the older gas-filled tubes. The Coolidge tube technique is still in its infancy and is under patent by the General Electric Company. Hence, for a long time, the mostly German manufacturers in Europe still produce only gas-filled tubes, also called ion tubes.
An example of the Coolidge X-ray tube. Left, the cathode connection, with the two connections for the filament. Right, the anode connection.
With the new tube, the irradiation times are shortened to 3/5 of the time needed with a Helm-type gas-filled tube. The high voltage is around 100 kV, the same as could be reached with the Apex apparatus. The penetration of the radiation is, as far as Gaarenstroom's observations are concerned, the same. Only after 1919 are all three irradiation units in the AVL, refitted for use of the Coolidge tubes.