The invisible revealing radiation

110 years ago, Wilhelm Röntgen discovered X-rays

At the end of the 19th century, Wilhelm Conrad Röntgen was the rector of the University of Würzburg, in Germany, and an experimental physicist interested in the study of delicate phenomena. He was, for example, researching electricity in crystals, the elastic properties of rubber, and the effect of pressure on the viscosity of liquids. When doing experiments with cathode rays in vacuum tubes, in October and November 1895, discovered a new kind of radiation, which was propagated in a straight line, was invisible, traversed great thicknesses of matter, and sensitized photographic plates. “For brevity, I shall use the expression ‘rays’ and, to distinguish them from others with the same name, I will call them ‘X-rays'”, he wrote at the time. At the end of December, Röntgen drafted the article on the discovery and convinced the Würzburg Physical and Medical Society to accept it and to publish it quickly. He also printed dozens of reprints and dispatched them to important scientists, accompanied by radiographs of objects and of the hand of his wife, Anna Bertha. In a few weeks, the novelty was being discussed in the academies and even commented on in the more popular press. In 1896, about a thousand articles were published on the subject. But he soon felt himself to be a victim of his own action. “In a few days, I was sick of the whole thing. I was no longer able to recognize my own work in the reports. For me, photography was a means to an end, but it was transformed into the most important thing”, he said in a letter to his friend Ludwig Zehnder. The practical effect was immediate: medicine began to use the X-rays in diagnosis. Later on, other sectors started to use them in a large number of applications.

The discovery occurred when Röntgen was studying the phenomenon of the luminescence produced by cathode rays (interpreted today as a high-speed electron beam) in a Crookes tube. When he connected the tube to a source of high voltage, it would emit radiation that turned certain substances luminous. Afterwards, he discovered that the radiation would also blacken nearby photographic plates. Röntgen then placed various opaque objects between the device and the plate and noted that the rays would cross through these bodies. He did the same with his wife’s hand and produced the first X-ray in history. The X-rays appear because of the rapid slowing down of an electrical charge, which in turn causes the emission of electromagnetic radiation.  “There are numerous authors who claimed to have anticipated the discovery of X-rays, one way or the other, but no author was ever presented who had carried out work similar to Röntgen’s before him”, explains Professor Roberto de Andrade Martins, from the Physics Institute of the State University of Campinas (Unicamp), a specialist in the history of physics. The work earned Röntgen the first Nobel Prize in Physics, in 1901. He preferred to donate the money to the University of Würzburg.