Early History of X Rays by ALEXI ASSMUS The discovery of X rays in 1895 was the beginning of a revolutionary change in our understanding of the physical world. N THE WINTER of the year of his fiftieth birthday, and the year I following his appointment to the leadership of the University of Würzburg, Rector Wilhelm Conrad Roentgen noticed a barium platinocyanide screen fluorescing in his laboratory as he generated cathode rays in a Crookes tube some distance away. Leaving aside for a time his duties to the university and to his students, Rector Roentgen spent the next six weeks in his labora- tory, working alone, and sharing nothing with his colleagues. 10 SUMMER 1995 Wilhelm Conrad Roentgen (1845–1923). (Courtesy of AIP Emilio Segré Visual Archives) Three days before Christmas he that jolted the fin- brought his wife into his laborato- de-siècle disci- ry, and they emerged with a photo- pline out of its graph of the bones in her hand and of mood of finality, the ring on her finger. The Würzburg of closing down Physico-Medical Society was the first the books with to hear of the new rays that could ever more precise penetrate the body and photograph measurements, of its bones. Roentgen delivered the losing itself in de- news on the 28th of December 1895. bates over statistical Emil Warburg relayed it to the Berlin mechanics, or of try- Physical Society on the 4th of Janu- ing to ground all ary. The next day the Wiener Press physical phenomena in carried the news, and the day fol- mathematically precise lowing word of Roentgen’s discovery fluctuations of the ether. began to spread by telegraph around All three discoveries, X rays, the world. uranium rays, and the elec- On the 13th of January, Roentgen tron, followed from one of the presented himself to the Kaiser and major experimental traditions in the was awarded the Prussian Order of second half of the nineteenth the Crown, Second Class. And on the century, the study of the discharge 16th of January the The New-York of electricity in gases. All three Times announced the discovery as contributed to a profound transfor- a new form of photography, which mation of physics. In the 20th cen- revealed hidden solids, penetrated tury, the discipline has been ground- wood, paper, and flesh, and exposed ed in the study of elementary Forms of tube used by Roentgen the bones of the human frame. “Men particles. in 1895–1896 for the production of science in this city are awaiting As with the invention of in- of X rays. with the utmost impatience the candescent light arrival of English technical journals bulbs, the study which will give them the full par- of electrical dis- ticulars of Professor Roentgen’s dis- charge through covery of a method of photographing gases was made opaque bodies,” The New-York possible by the Times began, and it concluded by pre- development of dicting the “transformation of mod- improved vacu- ern surgery by enabling the surgeon um technology to detect the presence of foreign in the 1850s. Ear- bodies.” (Jan. 16, 1896, p. 9) ly on, English The public was enthralled by this scientists were new form of photography and curi- investigating the ous to know the nature of the new patterns of light rays. Physicians put it to immediate and dark that ap- use. Physicists sat up and took no- peared in sealed tice. The discovery of X rays was the lead-glass tubes. first in a series of three discoveries The patterns in German Museum, Munich BEAM LINE 11 Roentgen’s apparatus for studying the ionization of air by X rays, 1906. transparent to ultra-violet light. When Heinrich Hertz found that he could pass the rays through metal foil, a fellow German scientist, Philip Lenard, began to study them more carefully. Lenard designed a tube with a thin aluminum window through which the rays could emerge, and he measured how far German Museum, Munich they could travel and still induce these partially evacuated tubes were fluorescence. Defined in this way, stimulated by a voltage drop between the range of the cathode rays was six a cathode and an anode: typically to eight centimeters. Lenard’s ex- there was a dark space, called periments inspired Roentgen to won- Crookes’ dark space; then a glow, der if the rays in an attenuated form called negative light; then another really traveled farther, and he dark space, this one called Faraday’s; planned experiments to see if a and a final glow of positive light. If sensitive electroscope would mea- the air in the tube was exhausted un- sure a discharge at four times the til the first dark space expanded to distance Lenard had identified. fill the entire tube and all glows dis- This line of work was outside appeared, then the rays emitted from Roentgen’s usual research pursuits, the cathode could be investigated. which had by this time gained him The rays cast shadows, and were great stature in German science. Son deflected by magnetic fields, but of a cloth manufacturer and mer- appeared to be immune to the ef- chant from the Rhine province, fects of static electric forces. Roentgen was not a particularly As was to be characteristic of the diligent student in his youth. He new ray physics to come—the phys- eventually made his way to the ics of cathode rays, X rays, alpha rays, Polytechnic in Zurich, where he beta rays, gamma rays, and N rays— obtained a diploma in mechanical the nature of the cathode rays was in engineering in 1868 and a doctor- dispute, the British favoring a stream ate one year later. In Zurich he of particles, those on the Continent became an assistant to August Kundt preferring to think of them as some and moved along with him to the sort of disturbance of the ether. (The University of Würzburg, and then on British position, and the research pro- to the Physical Institute at Stras- gram developed by J.J. Thomson at bourg. His first move on his own was the Cavendish Laboratory to study to the chair of physics at Giessen ionization in gases, would result in in Hesse in 1879, from which he the discovery of the electron. But our received many offers to go elsewhere. story does not take us that way). The path upward in the German A strong reason for believing that university system was to follow calls the cathode rays were particles was to universities of higher and higher Sir Joseph John Thomson, 1856–1940. the observation that they would stature, and finally to obtain an (Courtesy of the AIP Niels Bohr Library) not pass through matter that was institute of one’s own. Roentgen 12 SUMMER 1995 refused the calls until the Universi- centimeters that Lenard had found ty of Würzburg offered him the to be the maximum distance for Directorship of their Physical Insti- which cathode rays maintain their tute. In 1894 he was elected Rector power to induce fluorescence. Roent- at Würzburg. In his inaugural ad- gen recognized the effect as wor- dress, given the year before his dis- thy of his undivided attention and covery of X rays, Roentgen stated devoted the next six weeks to its that the “university is a nursery of uninterrupted study. scientific research and mental edu- Historians have speculated about cation” and cautioned that “pride in why Roentgen was the first to rec- Phillip Lenard, 1862–1947. (Courtesy of one’s profession is demanded, but not ognize the significance of this effect. Ullstein Bilderdienst and the AIP Niels professional conceit, snobbery, or The equipment, a cathode ray tube Bohr Library) academic arrogance, all of which and a fluorescing screen, had been in grow from false egoism.”* use for decades. In 1894 J.J. Thomson Roentgen’s pride could rest in the had seen fluorescence in German- over forty papers he had published glass tubing several feet from the from Strasbourg, Giessen, and discharge tube. Others had noted Würzburg. These early interests fogged photographic plates. But ranged widely—crystals, pyroelec- before Lenard’s work, the object of trical and piezoelectrical phenomena, study was always the effects inside and the effects of pressure on liquids the tube itself, and stray ultra-ultra- and solids—but did not yet include violet light could be used to explain electrical discharges in gases. He had the fogging of photographic plates. taken his turn at measuring the Lenard’s great interest was in prov- Demonstration by Crookes that cathode specific heat ratios of gases using a ing, in contradiction to the British, rays travel in straight lines: a) cathode; sensitive thermometer of his own the ethereal nature of cathode rays, b) aluminum cross and anode; d) dark making. He was an exact experi- and he was the first to study the shadow; c) fluorescent image. menter who often made his own apparatus—a skill learned during his training as an engineer in Zurich— and he was able to measure ex- tremely small effects, surpassing even Faraday’s measurement of the rotation of polarized light in gases. Roentgen turned to a new interest in October of 1895: the study of cath- ode rays. In the course of repeating the experiments of Hertz and Lenard, he happened to notice a glowing flu- orescent screen set off quite some distance from the Crookes’ tube he was operating. The screen sat much farther away than the six to eight *Quoted in “Wilhelm Conrad Roentgen,” Dictionary of Scientific Biography (New York: Scribner’s, 1975), p.