English: An early Crookes x-ray tube from museum of Wilhelm Conrad Röntgen in Würzburg. These first generation 'cold cathode' tubes were used from the 1890s until about 1920. The electrode at the left is the cathode, which releases a stream of electrons which strike the angled anode at the center of the bulb, generating x-rays, which radiate through the rear wall of the glass envelope. The anode is a massive bar to dissipate the heat generated by the electron beam. The sausage-shaped glass bulb attached at the bottom is a 'softener' device, which controls the residual pressure in the tube. Crookes tubes required some gas in them to operate. However, over time the gas was absorbed by the walls of the tube, lowering the pressure, resulting in 'harder' x-rays, until the tube stopped working entirely. The 'softener' used various methods to release a small amount of gas into the tube, restoring it to operation.