page_title: Crookes tube
section_title: The discovery of X-rays
hierarchical_section_title: Crookes tube / The discovery of X-rays
caption_attribution_description: English: Drawing of a Crookes type x-ray tube made by Alfred Cossor, from the early 1900s. The caption text: 'A Cossor bulb with automatic softening device and fin radiator for cooling anticathode.' Alterations to image: cropped out caption. The electrode on the right is the aluminum cathode, which focuses a beam of electrons on a small (~1 mm) spot on the angled platinum anode target, called the 'anticathode', in the center of the bulb, creating x-rays. The anticathode is angled so the x-rays are radiated downwards, passing out through the glass side wall of the tube. The electrode at the 10 o'clock position is called the auxiliary anode. The sausage-shaped device at the top is an 'automatic softener' to control the pressure in the tube. Crookes type tubes required some gas in the tube to operate, but with time the residual gas was absorbed and the vacuum in the tube increased, requiring a higher potential to operate, generating 'harder' x-rays, until eventually the tube stopped operating. The 'softener' prevents this. When the pressure drops and the voltage across the tube increases, the anode potential arcs across the spark gap to the softener electrode, and the current heats the helical sleeve in the softener, which releases gas, raising the pressure in the tube. Alfred Charles Cossor's workshop in Clerkenwill, London was at that time the only British manufacturer of Crookes x-ray tubes. These cold cathode x-ray tubes were used until the 1920s.
context_page_description: A Crookes tube is an early experimental electrical discharge tube, with partial vacuum, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were discovered.
  Developed from the earlier Geissler tube, the Crookes tube consists of a partially evacuated glass bulb of various shapes, with two metal electrodes, the cathode and the anode, one at either end. When a high voltage is applied between the electrodes, cathode rays are projected in straight lines from the cathode. It was used by Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, Kristian Birkeland and others to discover the properties of cathode rays, culminating in J.J. Thomson's 1897 identification of cathode rays as negatively charged particles, which were later named electrons. Crookes tubes are now used only for demonstrating cathode rays.
  Wilhelm Röntgen discovered X-rays using the Crookes tube in 1895. The term Crookes tube is also used for the first generation, cold cathode X-ray tubes, which evolved from the experimental Crookes tubes and were used until about 1920.
context_section_description: When the voltage applied to a Crookes tube is high enough, around 5,000 volts or greater, it can accelerate the electrons to a high enough velocity to create X-rays when they hit the anode or the glass wall of the tube. The fast electrons emit X-rays when their path is bent sharply as they pass near the high electric charge of an atom's nucleus, a process called bremsstrahlung, or they knock an atom's inner electrons into a higher energy level, and these in turn emit X-rays as they return to their former energy level, a process called X-ray fluorescence. Many early Crookes tubes undoubtedly generated X-rays, because early researchers such as Ivan Pulyui had noticed that they could make foggy marks on nearby unexposed photographic plates. On November 8, 1895, Wilhelm Röntgen was operating a Crookes tube covered with black cardboard when he noticed that a nearby fluorescent screen glowed faintly. He realized that some unknown invisible rays from the tube were able to pass through the cardboard and make the screen fluoresce. He found that they could pass through books and papers on his desk. Röntgen began to investigate the rays full-time, and on December 28, 1895, published the first scientific research paper on X-rays. Röntgen
  was awarded the first Nobel Prize in Physics (in 1901) for his discoveries.
  The many applications of X-rays created the first practical use for Crookes tubes, and workshops began manufacturing specialized Crookes tubes to generate X-rays, the first X-ray tubes. The anode was made of a heavy metal, usually platinum, which generated more X-rays, and was tilted at an angle to the cathode, so the X-rays would radiate through the side of the tube. The cathode had a concave spherical surface which focused the electrons into a small spot around 1 mm in diameter on the anode, in order to approximate a point source of X-rays, which gave the sharpest radiographs. These cold cathode type X-ray tubes were used until about 1920, when they were superseded by the hot cathode Coolidge X-ray tube.
caption: Crookes X-ray tube from around 1910.
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