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Röntgen (Roentgen), Wilhelm Conrad


(b. Lennep im Bergischen [now part of Remscheid], Rhine Province, Germany, 27 March 1845; d. Munich, Germany, 10 February 1923)


Röntgen was the only son of Friedrich Conrad Röntgen, a cloth manufacturer and merchant of Lennep, who belonged to an old Lutheran Rhineland family. His wife, Charlotte Constanze Frowein, was born in Holland, although her family, too, came originally from Lennep. When their son was three, they moved to Appeldoorn in Holland, and here Röntgen attended a private boarding school, the Institute of Martinus Herman van Doorn.

He seems not to have been a particularly studious boy, preferring to be out-of-doors and to use his hands. There is some doubt concerning the exact course of Röntgen’s education until he entered the Utrecht Technical School in December 1862, at the age of sixteen. Apparently he was expelled from a school in Utrecht for refusing to identify a classmate who had caricatured one of the masters. The continuity of his formal progress toward the university was thus broken, and he was never accepted as a regular student by the University of Utrecht—to his own and his parents’ distress. After two-and-a-half years at the Technical School, and nine months’ attendance at the philosophy classes of the university, he passed an examination to enter the Polytechnic at Zurich, as a student of mechanical engineering.

Röntgen was extremely happy in Switzerland, both in his work and in his social life. He received his diploma as a mechanical engineer in 1868 and his doctor of philosophy degree a year later. With these qualifications, he became assistant to the professor of physics, August Kundt, whose friendship and support greatly furthered Röntgen’s career. While working in Zurich, Röntgen met his future wife, Anna Bertha Ludwig, the daughter of a German exile. In 1871 Röntgen accompanied Kundt to the University of Würzburg, and the following year he married Bertha Ludwig. The couple had no children, but they adopted Bertha Röntgen’s niece in 1887.

Würzburg saw the real beginning of Röntgen’s academic career, although at first he was disappointed because the university refused to give him any academic position, since he lacked the formal educational requirements. Shortly after his marriage, he moved to Strasbourg with Kundt, where he became tutor in the very fine Physical Institute. He spent the year 1875 as professor at the Agricultural Academy of Hohenheim, but he missed the excellent equipment at Strasbourg and soon returned there to teach theoretical physics. The series of papers he produced during the next two years resulted in his being offered the chair of physics at the University of Giessen, in Hesse. From 1879 to 1888 he worked at Giessen, building such a reputation that he was offered professorships at both Jena and Utrecht. He was not tempted to move, however, until the Royal University of Würzburg offered him the joint posts of professor of physics and director of the Physical Institute. In 1894 he became rector of the University of Würzburg. The following year Röntgen made his momentous discovery of X rays, which brought him international fame. He was made an honorary doctor of medicine of Würzburg in 1896, an honorary citizen of his birthplace, Lennep, and a corresponding member of the Berlin and Munich academies. On 30 November 1896 the Royal Society of London awarded jointly to Röntgen and Lenard the Rumford Medal. In 1900 Columbia University awarded Röntgen the Barnard Medal. The final accolades for this unassuming scientist were the erection of his statue on the Potsdam Bridge in Berlin, and the award, in 1901, of the first Nobel Prize for physics. He gave his prize money to further scientific studies at the University of Würzburg. In 1900, at the request of the Bavarian government, Röntgen moved from Würzburg to the chair of physics and the directorship of the Physical Institute at Munich.

Röntgen’s last years were shadowed by the distresses and privations of World War I. His wife died after a long illness in 1919, and in 1920 he retired from his chair at Munich. He spent a great deal of his time at his country house at Weilheim, near Munich, where he had an extensive library. He continued to work and to enjoy long country walks until the year before his death, which followed a short illness.

Röntgen’s early training as an engineer and his years as Kundt’s assistant at Würzburg, where there was no laboratory mechanic, formed his lifelong habit of making his own apparatus. He was, indeed, the meticulously conscientious experimenter. Röntgen invariably worked alone in the laboratory, and with nothing to disturb his concentration, he was able to develop acute powers of observation. He was able to detect and measure extremely small effects, for example, the compressibility of liquids and solids and the rotation of the plane of polarization of light in gases. His reticence caused him to shun public engagements, and he never acquired the requisite lecturer’s skills. He even declined to give the expected lecture when he won the Nobel Prize. Röntgen was well known for his assiduous reading of the scientific literature, yet he never allowed his retiring and studious nature to interfere with his university administrative duties. His attitude to his profession is clearly defined in the address that he gave in 1894, when he became rector of Würzburg University;

The University is a nursery of scientific research and mental education, a place for the cultivation of ideals for students as well as for teachers. Her significance as such is much greater than her practical usefulness, and for this reason one should endeavour, in filling vacant places, to choose men who have distinguished themselves as investigators and promotors of Science, and not only as teachers; for every genuine scientist, whatever his line, who takes his task seriously, fundamentally follows purely ideal goals and is an idealist in the best sense of the word. Teachers and students of the University should consider it a great honour to be members of this organization. Pride in one’s profession is demanded, but not professional conceit, snobbery or academic arrogance, all of which grow from false egotism [Glasser, 1933, p. 100].

In all, Röntgen wrote fifty-eight papers, some with collaborators. Most of them were published in Annalen der Physik und Chemie. The fifteen Strasbourg publications covered such topics as the ratio of the specific heats of gases, the conductivity of heat in crystals, and the rotation of the plane of polarization of light in gases. Four papers on this last subject were the result of his joint work with Kundt. It was only because of the very high level of their experimental skill that the phenomenon was able to be observed and measured, something that Faraday had not been able to achieve. During Röntgen’s professorship at Giessen, he published eighteen papers. Work on the relation between light and electricity was being done by Röntgen at much the same time as by Kerr, who discovered the effect that bears his name. As part of his lifelong interest in crystals, he studied pyroelectrical and piezoelectrical phenomena. Having constructed a very sensitive air thermometer, he was able to measure the absorption of heat in water vapor, and his flair for experiment was also shown by his work on the compressibility of liquids and solids.

Röntgen’s fame rests on two pieces of work, both of which were far outside his normal field of research. The discovery of X rays is the more famous, but the earlier one concerned the magnetic effects produced in a dielectric, such as a glass plate, when it is moved between two electrically charged condenser plates. He set himself to test the electromagnetic theory of Maxwell, which implies that there will be a magnetic field in a dielectric whenever the electric field changes. In 1878 Rowland claimed to have detected the magnetic effect caused by the motion of electrostatic charges, but others could not repeat the experiments. For Röntgen here was a challenge. In a paper published in 1888 he demonstrated beyond doubt both the reality of the effect and the ability of Maxwell’s theory to explain it quantitatively. H. A. Lorentz named the effect the “roentgen current,” and Röntgen himself considered it as having as much importance as his discovery of X rays because it led to the theories of Lorentz and is the basis for modern theories of electricity.

During the eleven years which he spent at Würzburg, Röntgen published eighteen papers, the final three embodying the discovery of X rays. The earlier papers dealt with the effects of pressure on the physical properties of solids and liquids. While professor at Munich, administrative work took up so much of his time that only seven papers were produced between 1900 and 1921. These were concerned with the physical properties of crystals, their electrical conductivity, and the influence of radiation on them. The investigations published in 1914 on pyroelectricity and piezoelectricity proved of particular significance in clarifying the real nature of these effects.

It is, of course, for the discovery of X rays, as he called them, that Röntgen is known to the general public. In Germany, the name given to the rays is more usually Röntgenstrahlen. It is now known that X rays are part of the electromagnetic spectrum, as is light. The wavelengths of X rays are very short, occupying the region 0.01 to 50 angstroms.

On Friday, 8 November 1895, Röntgen first suspected the existence of a new phenomenon when he observed that crystals of barium platinocyanide fluoresced at some distance from a Crookes tube with which he was experimenting. Again, this investigation into gas discharges was outside his normal field of interest. Hertz and Lenard had published on the penetrating powers of cathode rays (electrons), and Röntgen thought that there were unsolved problems worth investigation. He found time to begin his repetition of their experiments in October 1895. As a preliminary to viewing the cathode rays on a fluorescent screen, Röntgen completely covered his discharge tube with a black card, and then chanced to notice that such a screen lying on a bench some distance away was glowing brightly. Although others had operated Crookes tubes in laboratories for over thirty years, it was Röntgen who found that X rays are emitted by the part of the glass wall of the tube that is opposite the cathode and that receives the beam of cathode rays. He spent six weeks in absolute concentration, repeating and extending his observations on the properties of the new rays. He found that they travel in straight lines, cannot be refracted or reflected, are not deviated by a magnet, and can travel about two meters in air. He soon discovered the penetrating properties of the rays, and was able to produce photographs of balance-weights in a closed box, the chamber of a shotgun, and a piece of nonhomogeneous metal; he also noticed the outlines of the bones in his fingers on these photographs. The apparent magical nature of the new rays was something of a shock even to Röntgen, and he, naturally, wished to be absolutely sure of the repeatability of the effects before publishing. On 22 December he brought his wife into the laboratory and made an X-ray photograph of her hand. It was no doubt the possibility of seeing living skeletons, thus pandering to man’s morbid curiosity, that contributed to the peculiarly rapid worldwide dissemination of the discovery. The first communication on the rays, on 28 December, was to the editors of the Physical and Medical Society of Würzburg, and by 1 January 1896 Röntgen was able to send reprints and, in some cases, photographs to his friends and colleagues. Emil Warburg displayed some of the photographs at a meeting of the Berlin Physical Society on 4 January. The Wiener Presse carried the story of the discovery on 5 January, and on the following day the news broke around the world. The world’s response was remarkably swift, both the general public and the scientific community reacting in their characteristic ways. For the former, the apparent magic caught the imagination, and for the latter, Crookes tubes and generators were promptly sold in great numbers.

After a royal summons, Röntgen demonstrated the effects of X rays to the Kaiser and the court on 13 January. He was immediately awarded the Prussian Order of the Crown, Second Class.

In March 1896, a second paper on X rays was published, and there followed a third in 1897, after which Röntgen returned to the study of the physics of solids. He had shown clearly the uses of the new rays for medicine and metallurgy, and so founded radiology, but the discovery of the nature of the rays and other applications he left to others. The hypothesis that X rays were transverse electromagnetic rays was proved by the experiments of Friedrich and Knipping, based on Laue’s idea of using a crystal as a diffraction grating. The possibility of an X-ray spectrometer was developed brilliantly by Moseley, whose papers of 1913 and 1914 showed the physical significance of atomic numbers and predicted the existence of three undiscovered elements.

X rays must have been produced by others long before Röntgen, probably with some of the electrical apparatus used during the eighteenth century. Crookes himself, in 1879, complained of fogged photographic plates that happened to be stored near his cathode-ray tubes. A. W. Goodspeed and W. N. Jennings in Philadelphia in 1890 noticed a peculiar blackening of photographic plates after having demonstrated a Crookes tube, but they failed to follow up their observation. Lenard and some other German physicists had noticed the fluorescence near Crookes tubes, but since they were concentrating on studying the properties of cathode rays, the strange side effects were not examined.


Röntgen’s publications are listed in Otto Glasser, Wilhelm Conrad Röntgen and die Geschichte der Röntgenstrahlen (Berlin, 1931; 2nd ed. 1959); published in English as Wilhelm Conrad Röntgen and the History of the Roentgen Rays (London, 1933). This work lists some three dozen books and articles concerning Röntgen, and over 1,000 books and pamphlets on X rays published in the year 1896.

The Deutsches Röntgen Museum at Remscheid-Lennep contains Röntgen’s personal possessions, library, photographic slides, private correspondence, and reminiscences of him recorded on magnetic tape.

W. Robert Nitske, The Life of Wilhelm Conrad Röntgen. Discoverer of the X Ray (Tucson, 1971), although it was intended as a popular biography, contains a six-page bibliography that includes papers concerned with the history of the discovery.

G. L’E. Turner

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Wilhelm Conrad Röntgen

Wilhelm Conrad Röntgen

For the first two decades of his scientific career, Wilhelm Conrad Röntgen (1845-1923) studied a fairly diverse variety of topics, including the specific heats of gases, the Faraday effect in gases, magnetic effects associated with dielectric materials, and the compressibility of water. He is most famous, however, for his discovery in 1895 of X rays, which had a revolutionary effect not only on physics but also on a number of other areas, particularly medicine, and for this he was awarded the first Nobel Prize in physics in 1901.

Wilhelm Conrad Röntgen was born in Lennep, Germany, on March 27, 1845. He was the only child of Friedrich Conrad Röntgen and the former Charlotte Frowein. His father was a textile merchant who came from a long line of metal workers and cloth merchants. His mother had been born in Lennep but then moved with her family to Amsterdam, where they had become wealthy as merchants and traders. When Röntgen was three years old, his family moved to Apeldorn, Holland. Otto Glasser speculates in Dr. W. C. Röntgen that the revolution of 1848 may have been a factor in this move because the family lost its German citizenship on May 23, 1848, and became Dutch citizens a few months later. In any case, Röntgen received his primary and secondary education in the public schools of Apeldorn and at a private boarding school in Middelann.

In December 1862, Röntgen enrolled at the Utrecht Technical School. His education at Utrecht was interrupted after about two years, however, when a childish prank went awry. He confessed to having drawn a caricature of an unpopular teacher for which another student had been responsible. As punishment, Röntgen was expelled from school, and his education was stalled until January 1865, when he was given permission to attend the University of Utrecht as an irregular student. There he attended classes on analysis, physics, chemistry, zoology, and botany. His future still seemed bleak, however, and, according to Glasser, "both Wilhelm and his parents had become resigned to his seeming inability to adjust to the requirements of the Dutch educational system and to obtain the credentials necessary to become a regular university student."

A friend of Röntgen's told him about the liberal admission policies at the Swiss Federal Institute of Technology in Zurich. Röntgen applied and was admitted at Zurich, and he arrived there to begin his studies in the mechanical technical branch of the institute on November 16, 1865. Over the next three years, Röntgen pursued a course of study that included classes in mathematics, technical drawing, mechanical technology, engineering, metallurgy, hydrology, and thermodynamics. On August 6, 1868, he was awarded his diploma in mechanical engineering. His degree had come in spite of his rather irregular attendance at classes. He later told Ludwig Zehnder that the lake and mountains surrounding Zurich were "too tempting." As a result, he became a devoted mountain climber and boater but an undistinguished student. Only when one of his professors told Röntgen that he would fail his examinations did he settle down to his studies.

At Zurich, the most important influence on Röntgen was the German physicist August Kundt. Kundt suggested to him that he do his graduate studies in physics rather than engineering, and Röntgen took his advice. On June 22, 1869, he was granted his doctoral degree for a thesis entitled "Studies about Gases." Kundt then asked him to become his assistant, an offer he quickly accepted. A year later, when Kundt was offered the chair of physics at the University of Würzburg in Germany, he brought Röntgen with him as his assistant.

While still in Zurich, Röntgen had met his future wife, Anna Bertha Ludwig, the daughter of a German revolutionary who had emigrated to Switzerland. They were married on January 19, 1872, after his move to Würzburg. The couple never had children of their own, although in 1887 they did adopt his wife's six-year-old niece Josephine Bertha.

After two years at Würzburg, Kundt moved once more, this time to the newly established University of Strasbourg in France. Again, he asked Röntgen to accompany him as his assistant. At Strasbourg, in March 1874, Röntgen finally achieved a long-delayed ambition: He was appointed a privatdozent at the university, his first official academic appointment. The appointment was the result of more liberal policies at Strasbourg; his lack of the necessary credentials had prevented him from receiving a formal appointment in any German university.

In 1875, Röntgen accepted a position as professor of physics at the Hohenheim Agricultural Academy. Missing the superb research facilities to which he had become accustomed in Strasbourg, however, he returned there in 1876 as associate professor of physics. Three years later he was appointed professor of physics at the University of Giessen in Germany, where he remained until 1888. He then returned to the University of Würzburg to take a joint appointment as professor of physics and director of the university's Physical Institute. Röntgen would remain at Würzburg until 1900, serving as rector of the university during his last six years there.

Röntgen wrote forty-eight papers on a diverse range of phenomena including the specific heats of gases, the heat conductivity of crystals, the Faraday and Kerr effects, the compressibility of solids and liquids, and pyroelectricity and piezoelectricity. Probably his most significant contribution during this period was a continuation of research originally suggested by James Clerk Maxwell's theory of electromagnetism. That theory had predicted that the motion of a dielectric material within an electrostatic field would induce a magnetic current within the dielectric material. During his last year at Giessen, Röntgen completed studies that confirmed this effect, a phenomenon for which Hendrik Lorentz suggested the name "röntgen current."

Yet there is no doubt that the discovery for which Röntgen will always be most famous is that of X rays. In 1894 Röntgen began research on cathode rays, which was then one of the most popular topics in physics. Much of the fundamental research on this topic had been carried out in the 1870s by the English physicist William Crookes. Crookes had found that the discharge of an electrical current within a vacuum tube produces a beam of negatively charged rays that causes a fluorescence on the glass walls of the tube. A number of scientists had followed up on this research, trying to discover more about the nature and characteristics of Crookes's cathode rays.

After repeating some of the earlier experiments on cathode rays, Röntgen's own research took an unexpected turn on November 8, 1895. In order to observe the luminescence caused by cathode rays more clearly, Röntgen darkened his laboratory and enclosed the vacuum tube he was using in black paper. When he turned on the apparatus, he happened to notice that a screen covered with barium platinocyanide crystals about a meter from the vacuum tube began to glow. This observation was startling, because Röntgen knew that cathode rays themselves travel no more than a few centimeters in air. It was not they, therefore, that caused the screen to glow.

Over the next seven weeks, Röntgen attempted to learn as much as he could about this form of energy. He discovered that its effect could be detected at great distances from the vacuum tube, suggesting that the radiation was very strong. He learned that the radiation passed easily through some materials, such as glass and wood, but was obstructed by other materials, such as metals. At one point, he even saw the bones in his hand as he held out a piece of lead before it. He also discovered that the radiation was capable of exposing a photographic plate. Because of the unknown and somewhat mysterious character of this radiation, Röntgen gave it the name X strahlen, or X rays.

On December 28, 1895, seven weeks after his first discovery of X rays, Röntgen communicated news of his work to the editors of a scientific journal published by the Physical and Medical Society of Würzburg. Six days earlier, he had made the world's first X-ray photograph, a picture of his wife's hand. Within weeks, news of Röntgen's discovery had reached the popular press, and the general public was captivated by the idea of seeing the skeletons of living people. On January 13, 1896, Röntgen was ordered to demonstrate his discovery before the Prussian court and was awarded the Prussian Order of the Crown, Second Class, by the Kaiser.

Röntgen actually devoted only a modest amount of attention to his momentous discovery. He wrote two more papers in 1896 and 1897, summarizing his findings on X rays, and then published no more on the subject. Instead, he went back to his work on the effects of pressures on solids. Röntgen chose not to ask for a patent on his work and refused the Kaiser's offer of an honorific "von" for his name. He did, however, accept the first Nobel Prize in physics, awarded to him in 1901. Even then, however, he declined to make an official speech and gave the prize money to the University of Würzburg for scientific research. His discovery had generated a surprising number of personal attacks, with many dismissing it as an accident or attributing it to other scientists. Glaser speculates that "Röntgen's reticence, bordering on bitterness with advancing years, was doubtless a defense against these attacks."

Röntgen had declined offers from other universities for many years, but in 1900, at the special request of the Bavarian government, he abandoned his chair at Würzburg in order to accept a similar position at the University of Munich. The decision was not an easy one for Röntgen because, as Zehnder later noted, "the nice quiet laboratory at Würzburg suited him so well." Röntgen remained at Munich until 1920 when he retired, a decision he made at least partly because of his grief over his wife's death a year earlier. She had suffered from a lingering disorder during which she became addicted to morphine. Zehnder was later to write that she was always "Röntgen's most understanding and truest friend."

Germany's defeat in World War I also had its effect on Röntgen: The inflationary period following the war resulted in his bankruptcy. He spent the last few years of his life at his country home at Weilheim, near Munich. He died there on February 10, 1923, after a short illness resulting from intestinal cancer. Among the many awards given to him were the Rumford Medal of the Royal Society (1896), the Royal Order of Merit, Bavarian (1896), the Baumgaertner Prize of the Vienna Academy (1896), the Elliott-Cresson Medal of the Franklin Institute (1897), the Barnard Medal of Columbia University (1900), and the Helmholtz Medal (1919).

Further Reading

Daintith, John, et al., A Biographical Encyclopedia of Scientists, Facts on File, Volume XX, 1981, p. 686.

Dibner, Bern, Wilhelm Conrad Röntgen and the Discovery of X-Rays, 1968.

Esterer, Arnulf K., Discoverer of X-Ray: Wilhelm Conrad Röntgen, 1968.

Gillispie, C. C., editor, Dictionary of Scientific Biography, Volume 1, Scribner, 1975, pp. 529-531.

Glasser, Otto, W. C. Röntgen and the Early History of Röntgen Rays, Charles C. Thomas, 1934.

Magill, Frank N., editor, The Nobel Prize Winners—Physics, Volume 1, 1901-1937, Salem Press, 1989, pp. 23-32.

Nitske, Robert W., The Life of W. C. Röntgen, Discoverer of the X-Ray, University of Arizona Press, 1971.

Wasson, Tyler, editor, Nobel Prize Winners, Wilson, 1987, pp. 879-882.

Weber, Robert L., Pioneers of Science: Nobel Prize Winners in Physics, American Institute of Physics, 1980, pp. 7-9.

Zehnder, Ludwig, Wilhelm Conrad Röntgen, Basle University, 192?. □

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Roentgen, Wilhelm Conrad

Wilhelm Conrad Roentgen (both: rĕnt´gĬn, rŭnt´–, Ger. vĬl´hĕlm kôn´rät rönt´gən), 1845–1923, German physicist. His notable research in many fields of physics, especially thermology, mechanics, and electricity, has been overshadowed by his discovery (1895) of a short-wave ray, the Roentgen ray, or X ray, for which he received the first Nobel Prize in Physics (1901). He taught at several German universities, including those at Würzburg (1888–99) and Munich (1899–1920).

See biography by W. R. Nitske (1971).

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Röntgen, Wilhelm Conrad

Wilhelm Conrad Röntgen: see Roentgen.

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