English: An early Crookes x-ray tube on display in the lobby of the Science Building at Winthrop University. These first generation 'cold cathode' tubes were used until about 1920. The electrode on the right is the cathode or negative electrode. The slanted electrode in the center of the bulb is the platinum anode or target. The electrode to the left is called the auxiliary anode. A high voltage applied between the cathode and anode ionized the small amount of residual air in the tube, creating cathode rays (electrons) at the cathode, which were accelerated and struck the anode, creating x-rays. The x-rays radiated through the side wall of the tube.