German physicist (1845–1923)

Röntgen, who was born at Lennep in Germany, received his early education in the Netherlands; he later studied at the Federal Institute of Technology, Zurich. After receiving his doctorate in 1869 for a thesis on States of Gases, he held various important university posts including professor of physics at Würzburg (1888) and professor of physics at Munich (1900). Röntgen researched into many branches of physics including elasticity, capillarity, the specific heat of gases, piezoelectricity, and polarized light. He is chiefly remembered, however, for his discovery of x-rays made at Würzburg on 8 November 1895.

In 1894 Röntgen had turned his attention to cathode rays and by late 1895 he was investigating the fluorescence caused by these rays using a Crookes tube. In order to direct a pencil of rays onto a screen, he covered a discharge tube with black cardboard and operated it in a darkened room. Röntgen noticed by chance a weak light on a nearby bench and found that another screen, coated with barium platinocyanide, was fluorescing during the experiment. He had already established that cathode rays could not travel more than a few centimeters in air, and as the screen was about a meter from the discharge tube he realized that he had discovered a new phenomenon. During the succeeding six weeks he devoted himself, feverishly and exclusively, to investigating the properties of the new emanations, which, because of their unknown nature, he called ‘x-rays’. On 28 December 1895 he announced his discovery and gave an accurate description of many of the basic properties of the rays: they were produced by cathode rays (electrons) at the walls of the discharge tube; they traveled in straight lines and could cause shadows; all bodies were to some degree transparent to them; they caused various substances to fluoresce and affected photographic plates; they could not be deflected by magnetic fields. Röntgen concluded that x-rays were quite different from cathode rays but seemed to have some relationship to light rays. He conjectured that they were longitudinal vibrations in the ether (light was known to consist of transverse vibrations). Their true nature was finally established in 1912.

Röntgen's discovery immediately created tremendous interest. It did not solve the contemporary wave–particle controversy on the nature of radiation but it stimulated further investigations that led, among other things, to the discovery of radioactivity; it also provided a valuable tool for research into crystal structures and atomic structure, and x-rays were soon applied to medical diagnosis. Unfortunately their danger to health only became understood very much later; both Röntgen and his technician suffered from x-ray poisoning.

Although Röntgen was subjected to some bitter attacks and attempts to belittle his achievements, his discovery of x-rays earned him several honors, including the first ever Nobel Prize for physics (1901).