xxxxxThe German physicist Wilhelm Roentgen discovered X-rays in November 1895 while experimenting with cathode rays. Although he had blocked off the rays being emitted from the Crookes tube, using a sheet of thin card, he suddenly saw that a piece of paper coated with a luminescent substance was giving off a glimmer of light. After further research he concluded that the tube was emitting invisible rays capable of penetrating not only card, but also wood and a thin layer of steel. He produced a paper on his find in January 1896, and within a few days this amazing discovery was known throughout the world. Its enormous value was quickly realised, not only in the field of medicine - where it could provide a picture of broken bones and vital organs - but also in other branches of science. For this discovery he was awarded the first Nobel Prize for physics in 1901. During his career, serving as professor of physics at Giessen, Wurzburg and Munich Universities, he also made valuable contributions in the study of mechanics, electricity and heat.

WILHELM ROENTGEN  1845 - 1923  (Va, Vb, Vc, E7, G5)


Roentgen: drawing by the artist Walter E. Hodgson, 1896, for The Windsor Magazine, a monthly illustrated journal, published in London from 1895 to 1939. X-ray: print, presented to the Swiss physicist Ludwig Zehnder (1854-1949), professor of physics at the University of Freiburg, Germany, in January 1896. Lenard: 1900, photographer unknown – Emilio Segrè Visual Archives, American Institute of Physics, College Park, Maryland. Birthplace: date and photographer unknown. Becquerel: date and photographer unknown, contained in the New Catholic Dictionary, 1910. Diagram: by courtesy of Explain That Stuff.

xxxxxIt was in November 1895, while experimenting with cathode rays, that the German physicist Wilhelm Roentgen discovered the highly penetrating radiation which came to be known as X-rays. This momentous discovery proved of vital practical importance as a diagnostic aid, not only in the field of medicine - where its enormous potential was quickly realised - but also in a number of other areas, including engineering for the testing of metals, and physics for determining crystal structures and analysing complex mixtures of elements.

xxxxxRoentgen was born in Lennep, Prussia, but with the coming of 1848, the Year of the Revolutions, the family moved to Apeldoom in the Netherlands (his mother being of Dutch descent), and it was here that he spent his childhood. He attended a private boarding school until 1861, and then studied at the technical college at Utrecht before taking a course in mechanical engineering at Zurich Polytechnic. Itxwas here that he was taught by August Kundt (1839-1894), the German physicist who made important advances in the study of sound and light. Kundt awakened his interest in physics, and after Roentgen had obtained a doctorate for his study of gases, he worked alongside Kundt as his assistant, first at Wurzburg University and then at the newly formed University of Strasbourg.

xxxxxIn 1874 he was appointed lecturer at Strasbourg and then, after serving as professor of physics and mathematics at the Agricultural Academy of Hohenehim, he returned to Strasbourg in 1876 to work once again alongside Kundt as an associate professor. Three years later he took up the chair of physics at the University of Giessen in Germany, and his experiments there, conducted over a period of eight years, confirmed his place as one of the leading German physicists. As a result, he was appointed professor and director of the Physics Institute at Wurzburg University in Bavaria in 1888 and elected Rector in 1894.


xxxxxIt was on the evening of the 8th November the following year, while studying the luminescence that cathode rays produced when passed through a variety of gases, that Roentgen stumbled upon a new, penetrating ray. Working in a darkened room, he suddenly observed that a sheet of paper coated with a luminescent substance called barium platinocyanide - lying some nine feet away - was giving off a glimmer of light. This mystified him because, as part of the experiment he was conducting at that time, he had deliberately blocked off the cathode rays being emitted from the Crookes tube (the forerunner of the cathode ray tube) with a sheet of thin black card. He then carried out further experiments and was forced to conclude that the tube was emitting invisible rays capable of penetrating a thick sheet of card, a piece of wood, photographic plates and even a thin layer of metal. And to prove the value of this new form of radiation he persuaded his wife Bertha to let him take an X-ray photograph of her left hand. After a 15 minute exposure she provided the first radiograph of a human being (here illustrated).


xxxxxIn January 1896 Roentgen read out a paper before the Physico-Medical Society of Wurzburg. Accepting the existence of these rays, but not understanding their make up, he used the term X-rays because in mathematics this letter is used to indicate the unknown. In the long term this became the generally accepted name, though in some countries, including Germany, they are still known as Roentgen Rays. He followed up this meeting with the publication of three papers entitled On a New Kind of Rays. The news of this fascinating discovery spread across the world in a matter of days. The idea that there existed a kind of invisible light that could penetrate wood and show the bones of a man’s hand was almost unbelievable. The London Standard, printing the news on the 7th January felt it necessary to assure its readers that “there is no joke or humbug in the matter. It is a serious discovery by a serious German professor”.

xxxxxAs one would expect, honours were showered upon Roentgen from across the world, and in January 1896 he travelled to Berlin to present his work to Emperor Wilhelm II. In the same year he received the Rumford Medal of the Royal Society of London. Hexshared this medal with the German Philipp Lenard (1862-1947) (illustrated), a physicist who had also carried out extensive research into cathode rays and produced a much-improved vacuum tube to assist in this work.

xxxxxIn 1900 Roentgen was appointed professor of the new Physics Institute at the University of Munich. After the First World War, with inflation spiralling out of control, he struggled to make ends meet and was eventually declared bankrupt. He retired in 1920, a year after Bertha’s death, and died three years later of stomach cancer. Regarded today as the father of diagnostic radiology, he was buried alongside his wife and parents in a cemetery at Giessen.

xxxxxDuring a lifetime devoted to science, Roentgen made a major contribution, to knowledge, particularly in the field of physics. He made advances in mechanics, electricity and heat, conducting research on a wide range of topics, including elasticity, the specific heat of gases, the capillary action of fluids, the conduction of heat in crystals, and polarized light.

xxxxxIncidentally, for reasons which are not clear Roentgen stated in his will that upon his death all his personal and scientific correspondence was to be destroyed. During his lifetime he refused to make any money out of his findings. He argued that any advances that were the result of scientific research belonged to humanity, and should be freely available to all. He also opposed the use of the term Roentgen Rays, but today the unit of radiation exposure is known universally as the roentgen. ……

xxxxx…… It is very likely that many scientists encountered X-rays in the course of their experiments but failed to identify them. It is known that the English physicist William Crookes often complained that he found photographic plates were foggy or darkened on taking them out of their boxes. This was almost certainly due to their exposure to X-rays during his experiments! ……

xxxxx…… As we have seen, the American inventor Thomas Edison played an important part in the development of the X-ray. In 1895 he made a study of materials which had the ability to fluoresce, and discovered that calcium tungstate was the best substance to use. This finding stepped up the use of the X-ray, particularly in its medical usage. X-rays were being used in the United States as early as 1896, and were soon employed in many countries to assist in dealing with bone fractures and gunshot wounds. ……

xxxxx…… Andxin 1897 the English scientist Joseph John Thomson (1856-1940) showed that cathode rays were electrons (he called them “corpuscles”) - subatomic elementary particles that carry a negative electric charge and are an integral part of matter. ……

xxxxx…… The house in Lennep where Roentgen was born (illustrated), situated 25 miles east of Dusseldorf, is now a museum dedicated to his life and work.





xxxxxA few months after Wilhelm Roentgen’s discovery of X-rays, the French physicist Henri Becquerel (1852-1908) discovered the phenomenon known as radioactivity - the spontaneous emission of radiation by a material. In 1886, after a lengthy study of luminescent materials, he found, virtually by chance, that uranium atoms, present in the salts he used in his experiments, emitted an entirely new kind of radiation. He studied further this radioactivity, and concluded that the atom had an internal structure and was not, in fact, the ultimate particle of matter. Research into radioactivity was then taken up by others. As we shall see, Pierre and Marie Curie found other radioactive materials in 1898 - polonium and radium - and a year later the New Zealand physicist Ernest Rutherford began his research into the disintegration of the elements and the chemistry of radioactive substances. Becquerel’s discovery, revolutionary in the extreme, paved the way for the study of nuclear physics, and marked the beginning of the nuclear age. For his research into spontaneous radioactivity he and Pierre and Marie Curie shared the Nobel Prize for Physics in 1903.

xxxxxA few months after the discovery of X-rays, the French physicist Henri Becquerel (1852-1908), taking Roentgen’s findings a step further, discovered - again by chance rather than design - the phenomenon known as radioactivity, the spontaneous emission of radiation by a material. In time this discovery, examined further, was to force scientists the world over to reassess their views on the structure of the atom, paving the way for the study of nuclear physics and the beginning of the nuclear age.

xxxxxBorn in Paris in 1852, Becquerel came from a family of physicians, the most notable being his grandfather Antoine-César Becquerel (1788-1878), one of the founders of electrochemistry, and his father, Alexandre-Edmond Becquerel (1820-91), the inventor of the phosphoroscope. Becquerel received his scientific training at the École Polytechnique in Paris and then studied engineering at the Bridges and Highways School in the city’s suburbs. He graduated in 1877 and worked for many years as an engineer in the Department of Bridges and Highways, ending his career as chief engineer in 1894. As a physicist he taught at the École Polytechnique and was elected to the Academy of Sciences in 1889. He was appointed professor of physics at the Museum of Natural history in 1892, and at the École Polytechnique three years later. His early research centred around polarized light and infrared radiation, and he also continued his father’s research into phosphorescence and fluorescence.

xxxxxEarly in 1896, intrigued by Roentgen’s experiments and already possessing a considerable knowledge concerning the emission of fluorescence, he began a study of luminescent materials in order to find out whether they emitted X-rays. Wrapping some photographic plates in black paper and putting the crystals of a fluorescent chemical on top - potassium uranyl sulphate - he placed the package in bright sunlight. On opening the packet several hours later he found that there was a greyish area on the plates. He concluded that the sun’s ultra-violent rays had induced fluorescence, and that the X-rays contained within had penetrated the black paper and “fogged” the plates. The following month, however, having left a second package, topped again with potassium uranyl sulphate, in a closed drawer - well shielded from any ultra-violet rays - he discovered to his surprise that the plates showed sharp outlines of the mineral samples. It was clear that fluorescence had played no part in this transfer. At this stage he concluded that salts of uranium were “particularly active”, but by May 1896, following further experiments, his discovery was complete, and he could confirm his findings. The new rays emerged from the element uranium. (The illustration shows one of the plates fogged by exposure to radiation from the uranium salts. The shadow of a metal Maltese cross, placed between the plate and the salts, is clearly visible).

xxxxxArmed with this information, Becquerel then began to study this new radiation, and by 1899 had discovered that, whilst it was similar to X-rays, it could be deflected by a magnetic field. This led him to the conclusion that part of it at least was made up of tiny charged particles, and that these electrons were coming from the atoms of uranium which had formed part of his fluorescent compound. That being the case, it was evident - revolutionary though it was - that the atom had an internal structure and was not the ultimate particle of matter.

xxxxxAs we shall see, research into radioactivity was then taken up by others. The French physicists Pierre and Marie Curie, for example, went in search of other radioactive materials, and this led to the discovery of polonium and radium in 1898 - both found in uranium ores. A year later the New Zealand physicist Ernest Rutherford, who became known as the father of nuclear physics and the founder of modern atomic theory, showed that radioactive material emits more than one kind of ray. He was awarded the Nobel Prize in Chemistry in 1908 “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances.”

xxxxxIn 1900 Becquerel isolated electrons in radiation and, the following year, he was the first to prove the phenomenon of radioactive transformation. And in 1901 his report of a burn, suffered when he carried an active sample of radium in his coat pocket, eventually led to the use of radium in medical treatment! His two major works were Research on Phosphorescence, and Discovery of the Invisible Radiation Emitted by Uranium. He was elected president of the French Academy of Sciences, and for his discovery of spontaneous radioactivity he shared the 1903 Nobel Prize for Physics with Marie and Pierre Curie. He died at Le Croisic, a small fishing port in Brittany, in August 1908, aged 55.

xxxxxIncidentally, in 1903 Becquerel supervised the work of Marie Curie when she was working for her doctorate at the University of Paris. ……

xxxxx…… The SI unit for radioactivity, the becquerel, is named after the French physicist, and he also has craters named after him on both the Moon and Mars. ……

xxxxx…… During his career he also researched into the physical properties of nickel, cobalt and ozone, and made a study of how crystals absorb light.