Wilhelm Conrad Röntgen
1845-1923
German Physicist
Wilhelm Röntgen wasn't allowed to graduate from high school because he got into trouble with a friend over a caricature of one of his teachers. But he won the first Nobel Prize for Physics in 1901 and had a very distinguished career in German universities. With a German father and Dutch mother, Röntgen grew up in Holland. Later, he went to a polytechnical school in Zurich, Switzerland, where he got a diploma in mechanical engineering. He married Anna Bertha Ludwig and was head of his departmentat the University of Würzburg when he noticed something important about the cathode rays with which he was working.
Sir William Crookes (1832-1919) had developed the Crookes tube in 1876 in which the pressure in a vacuum was reduced to the point that cathode rays shot straight across the tube and hit the wall opposite, causing it to glow with a greenish fluorescence. He noticed that wrapped and unexposed photographic plates left near his tubes became fogged, but he didn't understand why this happened. In 1895 Röntgen was experimenting with the Crookes tube in Würzburg, Germany, and observed the fluorescence of a barium platinocyanide screen that happened to be in its path. Putting things like books and cards between the tube and the wall seemed not to vary the effect. This suggested to Röntgen that it wasn't light or ultraviolet rays causing the fluorescence; it was found that x rays arise wherever cathode rays encounter solids and that the effect varies with the atomic weight of the target. The speed of the cathode particles also affects the penetrating effect of the x rays. X rays are a form of invisible, highly penetrating electromagnetic radiation with much shorter wavelengths or higher frequency than visible light. Their wavelength range is from less than a billionth of an inch to less than a trillionth of an inch. Scientists eventually realized that x rays are produced when high-energy electrons from a heated filament cathode strike the surface of a target.
Röntgen was greatly aided in his work by the newly emerging science of photography, which by the end of the nineteenth century was well advanced. Louis Daguerre and his colleagues had "fixed" an image from a camera obscura by adding mercury to the silver compounds being used, and the English inventor Fox Talbot (1800-1877) had also fixed his images on sensitized paper. These developments led to the production of "dry plates" in the 1870s and the emergence of the Kodak Company, which made cameras affordable to the general population. The first moving ("motion") picture was presented about the same time that Röntgen was making his observations about the effects of an unknown form of "radiation." Röntgen was an experimentalist: he experimented with different ideas and combinations of effects in order to understand what was happening, rather than forming a hypothesis from what was already known and then trying to confirm it in his laboratory. So he used photography to capture pictures of the effects of different experiments he undertook with the cathode rays of the Crooke's tube on different substances. This was possible because photography is the art of capturing different amounts of light as they impact on surfaces and substances, and Röntgen was interested in understanding this new form of creating fluorescent light on objects that were several feet away from the source of the energy. Röntgen explained his scientific process by saying "I didn't think, I investigated," but of course he was "playing around" with different techniques and technologies that had been developed in the later years of the nineteenth century. Using a form of Crooke's tube that had been adapted by his colleague at the University of Würzburg, Philipp Lenard (1862-1947; winner of the 1905 Nobel Prize for Physics for his work on electrons and atoms), Röntgen took a photograph of the skeleton of his wife's hand, with her wedding ring clearly visible but none of the flesh or veins. This photograph started the era of radioactivity.
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