Wilhelm Roentgen

Wilhelm Roentgen (1845-1923), German physicist, the first Nobel laureate in physics, which he achieved for his discovery of X-rays.

Wilhelm Conrad Roentgen was born at Lennep, in the lower Rhine Province, in Germany, on March 27, 1845, the only child of a merchant and manufacturer of cloth. At the age of three his family moved to the Netherlands. He attended Dutch secondary school and the University of Utrecht to study physics, but moved to the Zurich Polytechnic after passing the entrance examination to study mechanical engineering. There he was influenced by the great theoretical physicist Rudolf Julius Emanuel Clausius and August Adolph Kundt (1839-1894), the inventor of the simple device for measuring the velocity of sound in gases and solids that is named after him. In 1869, Roentgen graduated with a doctorate from the University of Zurich, was appointed assistant to Kundt, and eventually moved with him to the new University of Strasbourg. In 1874 he was appointed professor of physics and mathematics at the Academy of Agriculture, at Hohenheim, in Württemberg. He returned to Strasbourg as professor of physics in 1876, three years later moved into the same position at Giessen, and in 1888 he succeeded Friedrich Wilhelm Kohlrausch (1840-1910) as professor of physics and director of the new Physical Institute, at Würzburg. Among his colleagues were Hermann Ludwig Ferdinand von Helmholtz and Hendrik Antoon Lorenz (1853-1928). In 1900 he was appointed professor of physics at the University of Munich and the director of the Physical Institute. His fame established, he was offered, but declined, the presidency of the Physikalisch-Technische Reichsanstalt, in Berlin, and the chair of physics at the Berlin Academy.

Roentgen’s first work, published in 1870, dealt with the specific heat of gases, and he also investigated pyroelectrics and the piezoelectric effect, but his name will forever be associated with the discovery of “Roentgen” or X-rays (as he called them). This epoch-making discovery occurred on the evening of November 8, 1895, while experimenting with a Crookes tube in order to study the behaviour of electric currents through gases at extremely low pressure. This research was made possible because of the development of improved vacuum pumps, high-tension induction coils, and discharge tubes by the scientific glass-blower Heinrich Geissler, the physicist William Crookes, and others.

Roentgen observed in his dark laboratory the glowing of a fluorescent screen painted with barium platinocyanide placed in the path of the rays of a working Crookes tube which he had covered with black paper in order to exclude light. In a series of classical papers published between 1895 and 1897 he described the main properties of these unknown rays, which he therefore called X-rays. The picture that caught the scientific world by storm was the X-ray photograph of his wife’s hand, taken on December 22, 1895, which clearly showed the structure of the bones and the ring she was wearing. Roentgen demonstrated that the new rays were produced by the impact of cathode rays (electrons) on striking a target, which in Roentgen’s case was the glass envelope of the Crookes tube. It was left to Max von Laue (1879-1960) and his pupils to show that X-rays were a manifestation of highly energetic, invisible electromagnetic radiation.

Apart from the Nobel Prize in Physics awarded in 1901, many other honours were bestowed upon Roentgen. In 1872 he married Anna Bertha, a niece of the poet Otto Ludwig. Roentgen died in Munich on February 10, 1923, from intestinal cancer. The röntgen (R), the unit of radiation exposure, was named in his honour.