Wilhelm Conrad Röntgen was awarded the first Nobel Prize in Physics for his discovery of a new kind of radiation that was emitted by a cathode-ray tube.
Although atomism had been proposed by philosophers in ancient Greece, it was not placed on firm scientific foundations until the nineteenth century, beginning with the work of the English chemist John Dalton. During the nineteenth century, scientists investigated various properties of matter; in physics, this period was marked by extensive developments in the areas of thermodynamics, electricity, magnetism, and optics. Nevertheless, until the last decade of the century, the atom remained the ultimate, indivisible particle that its name implies. In the 1890’s, three discoveries were made that led eventually to the awareness that atoms have structure: the discovery of X rays by Wilhelm Conrad Röntgen in 1895, the discovery of radioactivity by Antoine-Henri Becquerel in 1896, and the discovery of electrons by Sir Joseph John Thomson in 1897.
The steps that led to the discovery of X rays began with the invention of the cathode-ray tube
Julius Plücker,
Röntgen began his auspicious academic career at the University of Utrecht in 1865, but he soon enrolled at the Swiss Polytechnic Institute in Zurich, Switzerland. There he worked under two famous physicists, Rudolph Clausius and August Kundt, the latter a brilliant experimentalist. After receiving the degree of doctor of philosophy in 1869 from the University of Zurich, Röntgen worked successively at the Universities of Würzburg, Strasbourg, and Giessen. Although his doctoral dissertation had been on the study of gases, he expanded his experimental work to research on pyro- and piezoelectric properties of crystals, surface phenomena of liquids, and dielectrics. In 1888, Röntgen returned to the University of Würzburg, having achieved a solid reputation as a physicist. By 1895, he had published forty-eight papers and had corresponded with eighty of the most prominent physicists of the time. In the 1890’s, Röntgen, like many of his contemporaries, decided to investigate phenomena associated with the cathode-ray tube.
On November 8, 1895, Röntgen was working alone in the physics institute at the University of Würzburg, attempting to determine the effect of covering a cathode-ray tube with a black cardboard box. He noticed that a paper treated with barium platinocyanide fluoresced, even though it was several meters away from the covered tube; as he was working at night and the laboratory was dark, he noticed the fluorescence easily. Lenard had discovered previously that cathode rays could penetrate a thin aluminum window at the end of a tube and cause fluorescence, but only up to a few centimeters from the tube. Aware of Lenard’s observation, Röntgen realized that he was observing a new, previously undetected kind of ray.
He was impressed with the intensity of the new rays and their penetrating power. Consequently, in his first experiments he placed various obstructions in the path of the rays, such as a book and wooden and metal objects. He noted that the degree of fluorescence was little reduced by wood or by a book of about one thousand pages but that lead and platinum blocked the rays completely. Röntgen then placed his hand in the path of the rays and saw that there was a clear outline of the bones on the fluorescent screen. Replacing the fluorescent screen with a photographic plate, he produced the first X-ray photograph. Unable to identify these new rays clearly, he called them simply “X rays” to indicate that their exact nature was as yet unknown; in Germany, they became known as Röntgen rays.
Wilhelm Conrad Röntgen.
The first public announcement of Röntgen’s discovery was made in a brief note to the Physico-medical Society of Würzburg. Within the medical community, there was immediate recognition of the significance of this discovery for medical diagnosis. In January, 1896, an X-ray photograph was made in Paris of a hand with a bullet embedded in it. So impressive was the photograph that popular as well as technical journals immediately reproduced it.
After the initial discovery, Röntgen measured the properties of X rays: their penetrating power, their ability to cause fluorescence, the refractive index in various media, and their reflectivity. As a result of further experiments, he distinguished between “hard” X rays, those that would penetrate very dense materials such as bones, and “soft” rays, those that would penetrate materials such as human flesh. In addition, he discovered that, although any solid body could be made to generate X rays when bombarded by the cathode rays, the most penetrating rays were produced when platinum was the target. When Thomson identified cathode rays as electrons in 1897, the basic mode of production of X rays—that is, by striking targets with high-velocity electrons—was clearly understood.
As is the case with so many other scientific discoveries, Röntgen’s discovery of X rays depended on a considerable amount of work done by his predecessors, but it is still reasonable to credit Röntgen with the clear identification of these rays. Nevertheless, because so many of his contemporaries also worked with cathode-ray tubes, it is not altogether surprising that some of them had, in fact, detected X rays without identifying them. The three physicists who did this were Crookes, A. W. Goodspeed, and Lenard.
In 1879, Crookes noted that photographic plates lying near his cathode-ray tube frequently became fogged, but his only reaction was to return the plates to the manufacturers as being faulty. In 1890, Goodspeed of the University of Pennsylvania had virtually the same experience, but he also failed to investigate the phenomenon further. Lenard, in his extensive work with cathode-ray tubes, undoubtedly dealt with X rays, but he failed to distinguish them from cathode rays, which were later identified as electrons. In view of the widespread reception of the discovery of X rays and its recognized significance, Röntgen was awarded the first Nobel Prize in Physics in 1901.
Röntgen’s discovery had an immediate and worldwide impact. The earliest record to appear in a scientific journal was published in the Electrical Engineer of New York on January 8, 1896, only two months after the discovery. In that year, more than a thousand books, pamphlets, and articles about X rays appeared. X rays’ mode of generation and their properties proved to be such fruitful areas for research that between 1896 and 1910, more than ten thousand publications were devoted to them.
Röntgen and his contemporaries recognized the significance of X rays as a diagnostic tool for medicine. In addition to using X rays to examine broken bones, physicians began to employ them for other diagnostic purposes, such as to examine the spinal column for defects, to examine the skull to determine the cause of fainting or blind spells, to detect metal objects in the body, and to detect cavities in teeth. X rays helped doctors detect chronic conditions such as arthritis, tuberculosis, and bone demineralization. Eventually, it was discovered that X rays can have serious negative effects on the human body. For example, they can damage reproductive cells, thus causing genetic diseases in subsequent generations.
Röntgen failed to determine the exact nature of X rays, a task that Max von Laue,
When X-ray spectra
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Beam Line: A Periodical of Particle Physics 25 (Summer, 1995). http://www.slac.stanford.edu/pubs/beamline/pdf/95ii.pdf. Special issue titled “One Hundred Years of X Rays” is devoted to discussion of the history and uses of X rays. Articles include “Early History of X Rays” and “Medical Applications of X Rays.” Features photographs, diagrams, and reproduced newspaper articles from the late nineteenth and early twentieth centuries. (Beam Line is published by the Stanford Linear Accelerator Center.) -
Bleich, Alan Ralph. The Story of X-Rays. New York: Dover, 1960. Provides a brief historical account of the discovery of X rays and their applications in medicine, art, crystallography, and industry. Intended for the general reader. Includes numerous illustrations, glossary, and index. -
Bragg, W. H., and W. L. Bragg. X-Rays and Crystal Structure. London: G. Bell & Sons, 1915. Written by the father-and-son team who pioneered the use of X rays to study the structure of crystals. Discusses the basic physics of X rays and crystals. Offers much information of historical interest. Includes many excellent photographs and illustrations. -
Dibner, Bern. Wilhelm Conrad Röntgen and the Discovery of X-Rays. New York: Franklin Watts, 1968. An excellent account of the discovery of X rays. Begins with the contributions made by Röntgen’s predecessors and the influence other physicists had on him. Intended for the general reader. Includes many fine illustrations and photographs. -
Michette, Alan, and Sawka Pfauntsch, eds. X-Rays: The First Hundred Years. New York: John Wiley & Sons, 1996. Collection of essays published in commemoration of the one hundredth anniversary of Röntgen’s discovery of X rays. Reviews the history of scientific work related to X rays as well as modern applications. Includes an extensive glossary. -
Thumm, Walter. “Röntgen’s Discovery of X Rays.” Physics Teacher 13 (April, 1975): 207-214. Shows why the common characterization of Röntgen’s discovery of X rays as an accident is not really tenable. Notes the care with which Röntgen examined the phenomenon and the failures of his contemporaries to make the discovery although they were working with the same equipment.
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