The Evolution of X-ray Tubes
Left: Crookes-Hittorf, similar to the one used by Wilhelm C. Röntgen during his discovery of ‘X-rays’. This type of tube also served Joseph J. Thomson in his discovery of the electron. Center: Gundelach tube, a two bi-anode cold cathode X-ray tube. Right: Coolidge tube, a high vacuum ‘hot cathode’ X-ray tube type still found in X-ray systems of today. (The three X-ray images were taken with a Nikon's XTH225-ST CT system housing a modern X-ray tube)
—Herminso Villarraga-Gómez, Nikon Metrology, Inc., Americas