Early x-ray tubes worked because vacuums were not good.  The ions of residual gas atoms colliding with the cathode caused a shower of electrons and these were attracted to the anode causing the generations of x-rays. With use the gas was absorbed in the tube wall and the x-rays reduced in intensity and became harder. It was necessary to introduce more gas if the tube was to stay useful and several devices (regulators) were invented to do this. Even so gas tubes were erratic in operation and they were superseded by tubes where electrons were generated by a heated filament. These thermionic tubes – invented by William Coolidge in 1913 –  were much more reliable and more powerful. They slowly took over, dominating the field after the early 1930s.

Early gas tube

Regulated gas tube

Coolidge tube

Cooled tube[5]


The History of Radiology, Adrian Thomas and Arpan Banerjee (G)

The Hooded Anode, Zahi Hakim  IL27 p12 (S)

Hakim’s website with images and pdf guide to contents

A brief history of the x-ray tube, Adrian Thomas IL15 p32  and as separate pdf