Rontgen, Wilhelm Conrad (physicist)

(1845-1923) German Experimental Physicist (Electrodynamics, Optics)

Wilhelm Conrad Rontgen discovered X rays in 1895, initiating a new era in physics and medicine. For this momentous achievement, he became the first recipient of the Nobel Prize in physics in 1901.

Rontgen was born in Lennep, in the Lower Rhine region of Germany, on March 27, 1845, the only child of a cloth manufacturer. Charlotte Constanze Frowein, his mother, was from Amsterdam, and, when Wilhelm was three, the family moved to Apeldoorn, the Netherlands, where he received his early education at a boarding school. As a boy, he loved to explore the countryside and showed an early aptitude for making mechanical devices. His boyhood was marred by the injustice of being expelled from the Utrecht Technical School for drawing a caricature of one of the teachers; the prank, in fact, had been committed by another student. He overcame this setback, however, and gained admission to the Federal Polytechnic Institute in Zurich, Switzerland, in 1866, to study mechanical engineering; there he received his diploma in 1868.

In 1869, Rontgen received a Ph.D. from the University of Zurich. While in Zurich, he met Anna Bertha Ludwig, the daughter of a cafe owner, and married her in 1872. They would not have children of their own; in 1887, they adopted the six-year-old daughter of Anna Rontgen’s only brother. In his immediate postdoctoral years, while serving as assistant to the German experimental physicist August Kundt, Rontgen decided to pursue pure science, specializing in physics. In order to be near Kundt, he held a series of positions, in Wurzburg and Strasbourg; in 1875, he became professor of physics and mathematics at the Agricultural Academy of Hohenheim. He returned to Strasbourg the following year to teach physics. From 1879 to 1888, he was professor of physics at Giessen. Then, in 1888, he became professor of physics at Wurzburg, where his colleagues included hermann ludwig ferdinand von helmholtz and hendrik antoon lorentz.

During this period, Rontgen carried out significant research on an impressive number of questions. He published his first paper, dealing with the specific heats of gases, in 1870. Between 1876 and 1895, he made important studies of the characteristics of gases and crystalline substances. These included thermal conductivity of gases, electrical and other characteristics of quartz, the influence of pressure on the refractive indices of various fluids, modification of the planes of polarized light by electromagnetic influences, variations in the behavior of the temperature and compressibility of water and other fluids, and the phenomena accompanying the spreading of oil drops on water.

However, this large body of work would be overshadowed by the great discovery Rontgen made in November 1895. He was studying the properties of cathode rays emitted by a partially evacuated glass Crookes tube (a glass vacuum tube with electrodes at each end). While experimenting with a barium compound, he was surprised to observe that it glowed even when the tube was encased in black cardboard. He took the chemical into another room; it continued to glow whenever the tube was activated. Rontgen concluded that these rays, with such high penetrating power, were entirely different from cathode rays. Later, when asked what his thoughts had been at the moment of his discovery, he said: “I didn’t think. I investigated.”

His investigations revealed that this new kind of cathode ray passed unchanged through cardboard and thin plates of metal, traveled in straight lines, and was not deflected by electric or magnetic fields. He called them X rays, since he could not explain their nature. Later max theodor felix von laue and his students would show that X rays possess the same electromagnetic nature as light but have a higher frequency of vibration. Using X rays, Rontgen took his first “X-ray photograph,” of his wife’s hand; it showed the shadows thrown by the bones of her hand and of the ring she was wearing, surrounded by the penumbra of the flesh, which threw a fainter shadow since it was more permeable to the rays.

In January 1896, after two months of thorough investigation the properties of X rays, he revealed his discovery to the general public. Almost immediately Rontgen’s announcement inspired further breakthroughs in both pure and applied science. Before long, development of X-ray equipment and photography for use in medical work was under way in Europe and the United States. Rontgen became the first recipient of the Nobel Prize in physics in 1901. He was showered with prizes, medals, honorary doctorates, and honorary and corresponding memberships to learned societies in Germany and abroad; several cities and streets were named after him.

In 1900, Rontgen became professor of physics and director of the Physical Institute at Munich, where he remained until he retired in 1920. He was a man of great integrity, uninterested in amassing personal wealth or honors. He refused the title von Rontgen, which would have made him a member of the German nobility, and donated the money for his Nobel Prize to the University of Wurtzburg. He did, however, accept the honorary degree of doctor of medicine offered to him at Wurzburg. Ironically, he never took out a patent on X rays and refused to benefit financially from the fruits of his discovery, since he believed that the products of scientific research should be available to everyone. He was almost bankrupt at the end of his life, during a period of runaway inflation in Germany that followed World War I. He died in Munich on February 10, 1923, of intestinal cancer.

Rontgen’s discovery of X rays had an enormous impact on the development of 20th-century physics. It led antoine-henri becquerel to discover radioactivity that same year, that discovery in turn caused a revolution in ideas about the atom. It would later lead to the discovery of X-ray diffraction, which yielded methods of studying atomic and molecular structure. In contemporary physics, X rays continue to find new uses in a growing number of fields, including condensed matter physics, molecular biophysics, astrophysics, nuclear physics, relativity, plasma physics, and cosmology.

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