English: Early Crookes x-ray tube from the Museum of Wilhelm Conrad Röntgen in Würzburg. The electrode at right is the cathode, which released electrons which hit the end of the anode, the angled copper rod in the center of the bulb, creating x-rays, which passed through rear wall of the tube. The anode was heated by the electron beam striking it, and the heat had to be dissipated or the anode could melt. The small round bulb to the lower left contained water for cooling the anode, and the anode is made of a hollow copper rod to conduct the heat efficiently to the water. The sausage shaped device attached just below the main bulb is an automatic 'softener', which controlled the vacuum in the tube. Crookes tubes required a small amount of air in them to function. With time, the air was absorbed by the walls of the tube, increasing the vacuum, until the tube stopped working. When that happened, a wire was attached to the softener, causing a current to pass through the black sleeve inside the softener, which released a small amount of gas into the tube, restoring the tube to function. These 'cold cathode' x-ray tubes were used until about 1920.