English: Drawing of a Crookes type x-ray tube made by Alfred Cossor, from the early 1900s. The caption text: 'A Cossor bulb with automatic softening device and fin radiator for cooling anticathode.' Alterations to image: cropped out caption.

The electrode on the right is the aluminum cathode, which focuses a beam of electrons on a small (~1 mm) spot on the angled platinum anode target, called the 'anticathode', in the center of the bulb, creating x-rays. The anticathode is angled so the x-rays are radiated downwards, passing out through the glass side wall of the tube. The electrode at the 10 o'clock position is called the auxiliary anode. The sausage-shaped device at the top is an 'automatic softener' to control the pressure in the tube. Crookes type tubes required some gas in the tube to operate, but with time the residual gas was absorbed and the vacuum in the tube increased, requiring a higher potential to operate, generating 'harder' x-rays, until eventually the tube stopped operating. The 'softener' prevents this. When the pressure drops and the voltage across the tube increases, the anode potential arcs across the spark gap to the softener electrode, and the current heats the helical sleeve in the softener, which releases gas, raising the pressure in the tube. Alfred Charles Cossor's workshop in Clerkenwill, London was at that time the only British manufacturer of Crookes x-ray tubes. These cold cathode x-ray tubes were used until the 1920s.