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Röntgen, Wilhelm Konrad

rays tube physics electrons

[ roent gen] (1845–1923) German experimental physicist: discoverer of X-rays.

Originally a student of engineering at Zürich Polytechnic, Röntgen was attracted to physics, which he studied and later taught in several German universities. He was professor of physics in Würzburg when he made his famous discovery, in 1895. While using a discharge tube (in which an electric discharge is passed through a gas at low pressure) in a darkened room, he noticed that a card coated with BaPt(CN)4 glowed when the tube was switched on. Röntgen soon found that the radiation causing this was emitted from the discharge tube at the region where the cathode rays (now known to be a stream of electrons) struck the glass end of the discharge tube. The new rays were found to have a much greater range in air than cathode rays; they travelled in straight lines and were not deflected by electric or magnetic fields; they passed through card, and even through thin metal sheet, and could be detected by a fluorescent screen or photographic plate. He named them X-rays. If passed through a human hand on to a photographic plate, the bones were seen as shadowed areas against the lighter flesh, and metal objects (for example a ring) gave opaque shadows. Röntgen suggested that the new rays were an electromagnetic radiation akin to light but of shorter wavelength, and this was proved in 1912.

Diagram of an X-ray tube. A current heats the tungsten filament to a temperature high enough for it to emit a stream of electrons. The potential difference VAC is large (> 20 000 volts) so the electrons are strongly attracted to it and strike the metal anode target at high speed, emitting X-rays. Ancillary equipment to evacuate the tube, water-cool the anode and protect the operator from X-rays is not shown.


Röntgen was awarded the first Nobel Prize in physics, in 1901, ‘for the discovery of the remarkable rays subsequently named after him’; in fact they are still known as X-rays. Their study added much to physics, gave a new technique for use in medicine and, after the work of the in 1915, led to X-ray crystallography as a new and immensely valuable method for the study of crystal and molecular structure. A modern X-ray tube uses a hot wire to generate electrons, which are accelerated by a high voltage and then strike a metal target, emitting X-rays (see diagram). Röntgen did excellent work on other areas of experimental physics. He took out no patents on his work, and died in some poverty in the period of high inflation in Germany.

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