how X-rays were discovered

The Mechanism

Wilhelm Conrad Röntgen was born on 27 March 1845 in Lennep, in the Rhineland, the only son of a cloth manufacturer. His family moved to Apeldoorn in the Netherlands when he was three; he was expelled from the Utrecht Technical School as a teenager for refusing to name a classmate who had drawn a caricature of a teacher, and could not therefore matriculate at a German university, so he took his diploma at the Federal Polytechnic Institute in Zürich in 1868 instead. By 1888, aged 43, he was Professor of Physics and Director of the Physical Institute at the Royal University of Würzburg in Bavaria — a modest institute in a two-storey building on the Pleicherring — and by 1894 he was also its Rector. Röntgen was a careful experimentalist of the old-fashioned German kind: methodical, painstaking, and famous for insisting on being present for every measurement. In the autumn of 1895 he was investigating the cathode rays that Philipp Lenard and Heinrich Hertz had reported passing through thin aluminium windows in evacuated glass tubes. On the evening of Friday, 8 November 1895, at the physics institute on the Pleicherring, Röntgen had a Hittorf-Crookes tube — a partially evacuated glass tube containing a cathode and anode across which he could pass a high-voltage discharge from a Ruhmkorff induction coil — running on his bench. He had wrapped the tube in thick black cardboard to exclude every visible light ray from the discharge, so he could check whether any cathode radiation escaped a second aluminium window at the far end. Somewhere on a shelf, a metre or two from the tube, sat a small paper screen he had prepared for a separate experiment. It was painted on one side with barium platinocyanide — Ba[Pt(CN)₄] — a phosphor that glows apple-green under ultraviolet or cathode-ray excitation. When Röntgen switched on the discharge, the shrouded tube emitted no visible light — but from across the room the screen began to fluoresce. He darkened the laboratory further. He moved the screen further from the tube; the glow persisted at almost two metres. He interposed a book of about a thousand pages between the tube and the screen; the glow was scarcely diminished. He interposed a plank of pine wood; the glow dimmed only a little. He interposed a sheet of one-and-a-half-millimetre lead; the glow ceased. Whatever the tube was emitting, it was passing through opaque solids at ranges no known form of light could match. Between 8 November and 22 December 1895 Röntgen barely left the institute; his wife Anna Bertha Ludwig (whom he had married in 1872) later said he ate and slept in the laboratory for those six weeks and refused all questions. He established that the new radiation travelled in straight lines, was not deflected by electric or magnetic fields, was absorbed by dense materials in rough proportion to their density and atomic weight, and could darken photographic emulsions through their light-tight paper wrappers. Because he did not know what the rays were, he named them, in the fashion of an algebraist, ***X-Strahlen*** — "X-rays" — with the letter denoting the unknown. On Sunday, 22 December 1895, Röntgen brought Anna Bertha into the laboratory. He placed her left hand between the tube and a photographic plate and exposed it for fifteen minutes. The developed plate showed her metacarpal bones, her phalanges, and — hovering above the shadow of her ring finger — the dark outline of her wedding band. When she looked at the plate, she said, *"Ich habe meinen Tod gesehen"* — "I have seen my death." Six days later, on 28 December 1895, Röntgen submitted a ten-page paper titled *Über eine neue Art von Strahlen (Vorläufige Mittheilung)* — "On a new kind of ray (preliminary communication)" — to the Physical-Medical Society of Würzburg, which printed it at once. He sent offprints with copies of the hand photograph to Franz Exner in Vienna, Arthur Schuster in Manchester, Lord Kelvin in Glasgow, Henri Poincaré in Paris, and half a dozen other leading physicists between New Year's Eve and 3 January 1896. Exner showed the print at a Vienna dinner party on 4 January; a guest gave it to the *Wiener Presse*, which ran the story on Sunday, 5 January; the *London Standard* cabled it worldwide on 6 January; by Wednesday, 8 January 1896, the discovery was on every front page in Europe. Within four months, the first diagnostic radiograph of a bullet in a human patient had been taken in Berlin. Röntgen took no patent on the discovery — he thought it belonged to humanity — and refused all requests to lecture on it commercially. In 1901 he received the first Nobel Prize in Physics ever awarded and donated the money to the University of Würzburg. It was another 17 years — until Max von Laue's 1912 experiments — before physicists knew that X-rays were high-energy electromagnetic waves, essentially light with wavelengths about ten thousand times shorter than visible light. Every hospital in the world contains a machine that Wilhelm Röntgen invented by accident, in a dark laboratory, on a Friday evening in November 1895.