BRIEF BIOGRAPHIES OF
WILHELM RÖNTGEN AND HENRI BECQUEREL

The last decade of the 19th century bore witness to the discovery of two phenomena so different from the range of normal experience that they could have been said to have started a new era in physics. These were the discovery of x-rays by Röntgen in 1895 and of natural (or spontaneous) radioactivity by Becquerel a few months later early in 1896. Both were found accidentally although Becquerel was influenced to some extent by the discovery of x-rays. Röntgen's discovery came about as a result of experiments and investigations on cathode rays, i.e., electrical discharge in gases, which, at the time was a topic of much research and interest. In retrospect, we can recognize the true significance of the discoveries of Röntgen and Becquerel, for together with those of Hertz and Thomson they heralded a new era in physics ... the transition from the classical to the modern age of physics. Without doubt, the final years of the 19th century were destined to become one of the most significant and productive periods in the whole history of science.


Wilhelm Konrad Röntgen was born in the small town of Lennep, Prussia, on March 27, 1845, the son of a well-to-do textile merchant. He grew up at a time very fertile with ideas, not only in physics but in a wide range of intellectual activities; Darwin's theory of natural selection in evolution, Pasteur's germ theory of disease, Pavlov and Freud in psychology, the philosophy of Nietzsche, William James and John Dewey, the literature of Mark Twain, Dostoevsky and Shaw, the art of Renoir, Cezanne and Rodin, the musics of Tchaikovsky and Brahms. All of these men were among his contemporaries in the last half of the 19th century; truly a time ripe for intellectual development.

When he was three years old Röntgen's family moved to Apeldoorn, Holland, and he attended public schools and, for a short time, a private boarding school. In his early schooling it does not seem that he impressed his teachers. After brief periods of study at the Utrecht Technical School and the University of Utrecht, he was admitted to the Polytechnic School in Zurich. He received a mechanical engineering diploma in 1868 but, being rather more intrigued by pure science after working for a time in the laboratory of August Kundt (1839-1894), he decided to study more mathematics and physics. After a year with Kundt, Röntgen was awarded a Ph.D degree from the University of Zurich [1] for his dissertation Studies on Gases. It was while he was at Zurich he developed a keen interest in mountain climbing, a hobby that he followed throughout his life.

Röntgen remained as an assistant to Kundt and went with him to the University of Wurzburg. In 1872 they went to the Kaiser-Wilhelm University of Strasbourg. Two years later, Röntgen was appointed a lecturer and he became an outstanding teacher and experimenter. He made significant contributions to a number of different fields; his range of interests and knowledge was the result of his extensive knowledge and understanding of the scientific literature obtained by studying the current journals. For example, in 1888 he proved that magnetic effects are produced in a dielectric when it is moved in an electric field, important because of its connection with the concepts of electromagnetism of Faraday and Maxwell. In October 1888 he accepted a position as Professor of Physics and Head of the Physical Institute at the University of Wurzburg, succeeding Friedrich Kohlrausch who had moved to the University of Strasbourg. He remained at Wurzburg for 12, highly productive, years confining himself completely to his academic and research duties. In fact, before discovering x-rays he had already published 48 scientific papers in various fields of research. In 1894 he was elected rector of the University. During the summer 1894 he started experimenting with a Hittorf-Crookes tube and studying the effects of cathode rays.

After Röntgen's tenure of office was over, in October 1895, he was able to devote more time to his studies. Then, late in the evening of Friday, November 8, 1895, he discovered a new type of radiation. He was working alone in his darkened laboratory with a Hittorf-Crookes tube enclosed in a black box when he noticed that a few crystals of barium platinocyanide that were scattered on the table began to fluoresce [2]. He deduced that the fluorescence could only have been caused by some hitherto unknown rays coming from the tube. Over the next few weeks he thoroughly examined the properties of the new rays and on December 2 he sent the manuscript of a paper entitled On a New Kind of Rays, a Preliminary Communication, to the secretary of the Physical-Medical Association in Wurzburg reporting his discovery. In the paper he says ... [the following extracts indicate how thoroughly Röntgen studied this new phenomenon] ... :

He tried blocks of wood, sheets of hard rubber, glass plates, various liquids and metals, discovering that ...

He demonstrated that lead-based paint also absorbed the new rays [3]. He also stated:

He investigated whether x-rays could be refracted through prisms of glass, various liquids, rubber and aluminum and concluded that in all cases the effect, if present at all, was very small so that the refractive indices must be less than 1.05. That meant:

He showed also that x-rays could not be deviated by a magnetic field and so concluded that

A little later in the initial manuscript he writes ... [the first reference to x-ray photographs and to the use of x-rays as a diagnostic tool] ... :

He tried, unsuccessfully, to detect interference effects. He wondered whether the x-rays were in fact ultra-violet light but he concludes that they are not. However, he says:

In a second paper, completed shortly after the first, Röntgen concentrates of the 'electrification' caused by x-rays and the discharge of electrified bodies.

The discovery of x-rays created an immediate sensation, not only because of its obvious scientific importance but also because of its diagnostic value in medicine. Within weeks of his initial announcement - copies of which had been circulated widely - and while his investigations were continuing, medical doctors were experimenting with x-rays. In a very short time x-rays were being used routinely in hospitals throughout the world. Röntgen demonstrated his discovery to the Kaiser and the court on January 13, 1896. (On the same day, it is reported that two British doctors used x-rays to locate a needle in a woman's hand.) Following a public lecture on January 23 he photographed the hand of one of the audience and frantic applause sprang up when the developed photograph was circulated. Röntgen was lauded with honors and awards; the most impressive of which was the first Nobel Prize in Physics, which he collected in Stockholm on December 10, 1901.

At the request of the Bavarian government Röntgen accepted the Directorship of the Physical Institute at Munich. The conditions at the University were not entirely to his liking and he became rather more interested and involved with the cultural attractions in Munich. It was this attachment to art and music that caused him to remain there until his death in 1923. He remained active in research until his retirement in 1920, but his fame and the many demands made on his time affected his work. Nevertheless, he trained many students, guided the affairs of his Department with skill and was constantly encouraging others in their research. The latter part of his life was marred somewhat by a controversy over the originality of his discovery of x-rays. Philip Lenard (1862-1947) at Heidelberg claimed that his own researches on cathode rays - which Röntgen acknowledged - had played a much more important role in the discovery than he had been given credit for. It seems there was no real basis for this claim but from time to time Lenard's colleagues attempted to get him more credit.

Early in 1896, within a few months of Röntgen's announcement of the discovery of x-rays, Becquerel made his equally staggering discovery of natural radioactivity (initially called Becquerel rays). Here, for the first time, was the evidence that the atom possessed structure (although it was some time before the connection was made). In fact, Becquerel's discovery made little impression at the time coming, as it did, so close to the excitement generated by the discovery of x-rays.


Antoine Henri Becquerel was born in Paris on December 15, 1852, the third in a line of distinguished scientists. His father, Alexandre Edmond, and grandfather, Antione Cesar, were both well-known physicists having each, in turn, occupied the Chair of Physics at the Musee d'Histoire Naturelle, and it is likely that they had a strong influence on Becquerel's choice of career. Following conventional early training Becquerel entered the Ecole Polytechnique and received the degree of Doctor of Science. In 1875 he entered government service as an engineer in the Department of Roads and Bridges, becoming Chief Engineer in 1894. At the same time he taught physics at the Musee where his father and grandfather had taught and following the death of his father in 1892 he succeeded to the same Chair that each of them had occupied previously. He was elected to membership in L'Institut de France in 1889 and in 1895 he was appointed Professor Physics at the Ecole Polytechnique.

Becquerel's early interests were concerned largely with various properties of light, such as polarization, absorption in crystals and phosphorescence. But it was while studying the latter that he discovered radioactivity. He was influenced by Röntgen's observations of fluorescence where the x-rays passed through the glass walls of the tube; he wondered whether there was a connection between x-rays and the fluorescence exhibited by certain materials, e.g., uranium minerals. So he decided to see if something similar to x-rays is emitted from these various compounds.

In an initial paper published in early 1896 he describes the following experiment:

Becquerel concluded:

He thought initially that the activity was caused by the action of sunlight on the phosphorescent substance itself, and in a further series of experiments measured the penetrative power of the radiation through sheets of aluminum and copper. He also showed that phosphorescence produced, not by direct sunlight, but by reflected and refracted light also caused the 'blackening' effect on the photographic plate. However, a fortuitous delay due to several days of poor weather caused him to discover the real cause. Becquerel describes it this way:

The latter experiment, in which all light was excluded, confirmed Becquerel's suspicions. In this first paper he was impressed by the similarity of his own radiations and those of Röntgen; he writes:

Later in 1896, following a number further investigations on a variety of different uranium compounds he makes two important discoveries:

Shortly after Becquerel's observations it was discovered that thorium had similar properties, which led to a systematic search by Marie and Pierre Curie to find these effects in other materials and culminated in the discovery in 1898 of natural radioactivity in polonium and radium.

Although radioactivity was initially discovered in 1896, the form of the radiation involved was not identified until 1899 when Ernest Rutherford (1871-1942) found that it consisted of two parts. He called the less-penetrative form alpha- and the more-penetrative form beta-rays. Later, the more penetrating gamma radiation was found generally to accompany the alpha- and beta-radiations. Becquerel showed that the beta-rays were easily deflected by a magnetic field and shortly after it was established that they were identical to cathode rays, i.e., fast moving electrons.

After his discovery of radioactivity in 1896 he continued to study their properties until his death in Brittany in 1908. He was awarded the Rumford Medal of the Royal Society and in 1903 shared the Nobel Prize in physics for "the discovery of spontaneous radioactivity" with Pierre and Marie Curie; the name of the phenomenom - radioactivity - had been suggested by Madame Curie.

FOOTNOTES

[1] The Polytechnic School and the University shared the same building and facilities.

[2] Fluorescence, or phosphorescence, occurs when the material continues to glow after the source has been removed.

[3] In a footnote in the manuscript Röntgen says: "For brevity's sake I shall use the expression 'rays'; and to distinguish them from other of this name I shall call them 'x-rays'". Although following his discovery it was proposed the rays be called 'Röntgen rays', the name x-rays has become universal.

[4] Crystals of uranyl and potassium bisulphates.

[5] Earlier in the paper he says ... "The phosphorescence of this substance is very strong, but its persistence is less than 1/100 of a second. The character of the luminous radiations emitted by this substance was studied at another time by my father ... "

REFERENCES

Books

M. Shamos Great Experiments in Physics (Dover Publications Inc., New York, 1987).

Web-sites