English: An early Crookes type X-ray tube. The electrode to the right is the cathode, which focused a narrow beam of electrons on the platinum or tungsten target at the center of the angled copper anode to the left, creating x-rays, which radiated downwards. The sausage-shaped bulb at top is a 'softener'. Crookes tubes required a certain amount of residual gas in them to function properly. As time passed, the gas was absorbed by the surfaces inside the tube, causing them to quit working. Applying a current through the 'softener' heated a mantle inside, releasing gas to restore the tube to operation. These types of x-ray tubes were used from the 1890s to about 1920.