Early History of X Raysys by ALEXI ASSMUS The The discovery ofX rays TEET吓阻 in 1895 was the THROUCH A BLOCK OF WOOD beginning of a “如 unl Ihe Goins wihin your PuIse revolutionary change ADMISSION 3d AL DAv in our understanding WONDERFUL NET X RAY PHOTOGRAPHS TAKEN of the physical world SEES THROUGH N THE WINTER of the year of his fiftieth birthday, and the year following his appointment to the leadership of the University of Wurzburg, Rector Wilhelm Conrad Roentgen noticed a barium platinocyanide screen fluorescing in his laboratory as he ARE 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 colleague 10 SUMMER 1995
Early History of X Rays by ALEXI ASSMUS 10 SUMMER 1995 The discovery of X rays in 1895 was the beginning of a revolutionary change in our understanding of the physical world. I N THE WINTER of the year of his fiftieth birthday, and the year 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 laboratory, working alone, and sharing nothing with his colleagues
Wilhelm Conrad Roentgen (1845-1923, (Courtesy of AlP Emilio Segre Visual Three days before Christmas he that jolted the fin- brought his wife into his laborato- de-siecle 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 wurzburg 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 al ary. The next day the wiener Press physical phenomena in carried the news, and the day fol- mathematically precise lowing word of Roentgens 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 resented 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 theThe 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 th 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. "(an. 16, 1896, p 9) ly on, English The public was enthralled by this scientist 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 BEAM LINE 11
BEAM LINE 11 Three days before Christmas he brought his wife into his laboratory, and they emerged with a photograph of the bones in her hand and of the ring on her finger. The Würzburg Physico-Medical Society was the first to hear of the new rays that could penetrate the body and photograph its bones. Roentgen delivered the news on the 28th of December 1895. Emil Warburg relayed it to the Berlin Physical Society on the 4th of January. The next day the Wiener Press carried the news, and the day following word of Roentgen’s discovery began to spread by telegraph around the world. On the 13th of January, Roentgen presented himself to the Kaiser and was awarded the Prussian Order of the Crown, Second Class. And on the 16th of January the The New-York Times announced the discovery as a new form of photography, which revealed hidden solids, penetrated wood, paper, and flesh, and exposed the bones of the human frame. “Men of science in this city are awaiting with the utmost impatience the arrival of English technical journals which will give them the full particulars of Professor Roentgen’s discovery of a method of photographing opaque bodies,” The New-York Times began, and it concluded by predicting the “transformation of modern surgery by enabling the surgeon to detect the presence of foreign bodies.” (Jan. 16, 1896, p. 9) The public was enthralled by this new form of photography and curious to know the nature of the new rays. Physicians put it to immediate use. Physicists sat up and took notice. The discovery of X rays was the first in a series of three discoveries that jolted the finde-siècle discipline out of its mood of finality, of closing down the books with ever more precise measurements, of losing itself in debates over statistical mechanics, or of trying to ground all physical phenomena in mathematically precise fluctuations of the ether. All three discoveries, X rays, uranium rays, and the electron, followed from one of the major experimental traditions in the second half of the nineteenth century, the study of the discharge of electricity in gases. All three contributed to a profound transformation of physics. In the 20th century, the discipline has been grounded in the study of elementary particles. As with the invention of incandescent light bulbs, the study of electrical discharge through gases was made possible by the development of improved vacuum technology in the 1850s. Early on, English scientists were investigating the patterns of light and dark that appeared in sealed lead-glass tubes. The patterns in Wilhelm Conrad Roentgen (1845–1923). (Courtesy of AIP Emilio Segré Visual Archives) Forms of tube used by Roentgen in 1895–1896 for the production of X rays. German Museum, Munich
Roentgens 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 enard, began to study them more carefully. Lenard designed a tube through which the rays could emerge, and he measured how far 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 Faradays: 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 Roentgens 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 Wurzburg, 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 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
12 SUMMER 1995 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 they could travel and still induce fluorescence. Defined in this way, the range of the cathode rays was six to eight centimeters. Lenard’s experiments inspired Roentgen to wonder if the rays in an attenuated form really traveled farther, and he planned experiments to see if a sensitive electroscope would measure a discharge at four times the distance Lenard had identified. This line of work was outside Roentgen’s usual research pursuits, which had by this time gained him great stature in German science. Son of a cloth manufacturer and merchant from the Rhine province, Roentgen was not a particularly diligent student in his youth. He eventually made his way to the Polytechnic in Zurich, where he obtained a diploma in mechanical engineering in 1868 and a doctorate one year later. In Zurich he became an assistant to August Kundt and moved along with him to the University of Würzburg, and then on to the Physical Institute at Strasbourg. His first move on his own was to the chair of physics at Giessen in Hesse in 1879, from which he received many offers to go elsewhere. The path upward in the German university system was to follow calls to universities of higher and higher stature, and finally to obtain an institute of one’s own. Roentgen these partially evacuated tubes were stimulated by a voltage drop between a cathode and an anode: typically there was a dark space, called Crookes’ dark space; then a glow, called negative light; then another dark space, this one called Faraday’s; and a final glow of positive light. If the air in the tube was exhausted until the first dark space expanded to fill the entire tube and all glows disappeared, then the rays emitted from the cathode could be investigated. The rays cast shadows, and were deflected by magnetic fields, but appeared to be immune to the effects of static electric forces. As was to be characteristic of the new ray physics to come—the physics of cathode rays, X rays, alpha rays, beta rays, gamma rays, and N rays— the nature of the cathode rays was in dispute, the British favoring a stream of particles, those on the Continent preferring to think of them as some sort of disturbance of the ether. (The British position, and the research program developed by J.J. Thomson at the Cavendish Laboratory to study ionization in gases, would result in the discovery of the electron. But our story does not take us that way). A strong reason for believing that the cathode rays were particles was the observation that they would not pass through matter that was Sir Joseph John Thomson, 1856–1940. (Courtesy of the AIP Niels Bohr Library) Roentgen’s apparatus for studying the ionization of air by X rays, 1906. German Museum, Munich
efused the calls until the universi- centimeters that Lenard had found y of Wurzburg 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 Wurzburg. 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- Philip Lenard, 1862-1947.(Courtesy of one's profession is demanded, but not ognize the significance of this effect. Ulstein Bilderdienst and the A/P 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 Thomson Roentgens 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 Wurzburg. These early interests fogged photographic plates. But anged 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 2b even Faradays measurement of the N rotation of polarized light in gas oentgen 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 cro jokes was operating. The screen sat much farther away than the six to eight Quoted in"Wilhelm Conrad Roentgen, Dictionary of Scientific Biography (New York: Scribners, 1975), p. 531 BEAM LINE 13
BEAM LINE 13 refused the calls until the University of Würzburg offered him the Directorship of their Physical Institute. In 1894 he was elected Rector at Würzburg. In his inaugural address, given the year before his discovery of X rays, Roentgen stated that the “university is a nursery of scientific research and mental education” and cautioned that “pride in one’s profession is demanded, but not professional conceit, snobbery, or academic arrogance, all of which grow from false egoism.”* Roentgen’s pride could rest in the over forty papers he had published from Strasbourg, Giessen, and Würzburg. These early interests ranged widely—crystals, pyroelectrical and piezoelectrical phenomena, and the effects of pressure on liquids and solids—but did not yet include electrical discharges in gases. He had taken his turn at measuring the specific heat ratios of gases using a sensitive thermometer of his own making. He was an exact experimenter who often made his own apparatus—a skill learned during his training as an engineer in Zurich— and he was able to measure extremely 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 cathode rays. In the course of repeating the experiments of Hertz and Lenard, he happened to notice a glowing fluorescent screen set off quite some distance from the Crookes’ tube he was operating. The screen sat much farther away than the six to eight centimeters that Lenard had found to be the maximum distance for which cathode rays maintain their power to induce fluorescence. Roentgen recognized the effect as worthy of his undivided attention and devoted the next six weeks to its uninterrupted study. Historians have speculated about why Roentgen was the first to recognize the significance of this effect. The equipment, a cathode ray tube and a fluorescing screen, had been in use for decades. In 1894 J.J. Thomson had seen fluorescence in Germanglass tubing several feet from the discharge tube. Others had noted fogged photographic plates. But before Lenard’s work, the object of study was always the effects inside the tube itself, and stray ultra-ultraviolet light could be used to explain the fogging of photographic plates. Lenard’s great interest was in proving, in contradiction to the British, the ethereal nature of cathode rays, and he was the first to study the Demonstration by Crookes that cathode rays travel in straight lines: a) cathode; b) aluminum cross and anode; d) dark shadow; c) fluorescent image. Phillip Lenard, 1862–1947. (Courtesy of Ullstein Bilderdienst and the AIP Niels Bohr Library) *Quoted in “Wilhelm Conrad Roentgen,” Dictionary of Scientific Biography (New York: Scribner’s, 1975), p. 531
effects of the rays in air or in a sec- shadowy pictures they produce ond glass tube into which he directed bones in a hand, a wire wrapped them around a bobbin, weights in a box, Roentgen, a meticulous and ob- a compass card and needle hidden rvant experimenter, made the away in a metal case, the inhomo- obvious tests on the new X rays: geneity of a metal. The ability of the Were they propagated in straight new rays to produce photographs lines? Were they refracted? Were they gave them great popular appeal and reflected? Were they distinct from brought Roentgen fame. Many arti cathode rays? What were they? Like cles appeared in photography jour- Heinrich Rudolf Hertz. 1857-1894 the cathode rays, they moved in nals, and The New - York Times in (Courtesy of Deutsches Museum and straight lines. Roentgen was unable dexed the new discovery under AIP Emilio Segre Visual Archives) to refract them with water and car- photography. Since the rays expose bon bisulphide in mica prisms. Nor photographic plate the public as- could he concentrate the rays with sumed they were some form of light ebonite or glass lenses With ebonite The physicist Roentgen concurred and aluminum prisms he noted the Accepting Lenard's claim that cath- possibility of refracted rays on a pho- ode rays were vibrations of the ether, tographic plate but could not observe Roentgen compared the new rays to this effect on a fluorescent screen. them and forwarded the opinion that Testing further, he found that X rays the two were ethereal, although dif- O Rontgen, then the news is true could pass freely through thick lay- ferent from visible, infra-red and And not a trick fide rumour ers of finely powdered rock salt, ultra-violet light in that they did not That bids us each beware of you. electrolytic salt powder, and zinc reflect or refract. He suggested that And of your grim and graveyard humour dust, unlike visible light which, cathode rays and X rays were longi We do not want like Dr: Swift. because of refraction and reflection, tudinal vibrations of the ether rather To take our fesh offand is hardly passed at all. He concluded than transverse ones ones or that X rays were not susceptible to Now that their existence was And joint for you to poke e your nose n egular refraction or reflection. established, it was easy enough to Roentgen found that the X rays experiment with the new X rays. We only crave to contemplate Each others usual full-dress photo originate from the bright fluores- Roentgen himself published only Your worse than altogether"state cence on the tube where the cathode three papers on the subject, but oth- Ofportraiture we bar in toto/ rays strike the glass and spread out. ers jumped quickly into the field The point of origin of the X rays And not just physicists. Thomas The fondest swain would scarcely prize moves as the cathode rays are moved Edison used modified incandescent A picture of his lady 's framework; by a magnetic field, but the X rays light bulbs to produce the new rays To gaze on this with yearning eyes Would probably be voted tame work/ themselves are insensitive to the He boasted to reporters that any magnet Roentgen concluded that one could make photographs of No, keep them for your epita they are distinct from cathode rays, skeleton hands; that was mere child,s these tombstone-souvenirs unpleasant, since Lenard's work had shown that play. Within a month of Roentgen Orgo away and photograph cathode rays passing through the announcement doctors were using Mahatmas, spooks, and Mrs. B-s-nt/ tube maintained their direction the X rays to locate bullets in human but were susceptible to magnetic flesh and photograph broken bones -Punch, January 25, 1896 deflection Dr Henry W. Cattell, Demonstrator Roentgen justified calling the new of Morbid Anatomy at the Univer phenomena rays because of the sity of Pennsylvania, confirmed their 14 SUMMER 1995
14 SUMMER 1995 effects of the rays in air or in a second glass tube into which he directed them. Roentgen, a meticulous and observant experimenter, made the obvious tests on the new X rays: Were they propagated in straight lines? Were they refracted? Were they reflected? Were they distinct from cathode rays? What were they? Like the cathode rays, they moved in straight lines. Roentgen was unable to refract them with water and carbon bisulphide in mica prisms. Nor could he concentrate the rays with ebonite or glass lenses. With ebonite and aluminum prisms he noted the possibility of refracted rays on a photographic plate but could not observe this effect on a fluorescent screen. Testing further, he found that X rays could pass freely through thick layers of finely powdered rock salt, electrolytic salt powder, and zinc dust, unlike visible light which, because of refraction and reflection, is hardly passed at all. He concluded that X rays were not susceptible to regular refraction or reflection. Roentgen found that the X rays originate from the bright fluorescence on the tube where the cathode rays strike the glass and spread out. The point of origin of the X rays moves as the cathode rays are moved by a magnetic field, but the X rays themselves are insensitive to the magnet. Roentgen concluded that they are distinct from cathode rays, since Lenard’s work had shown that cathode rays passing through the tube maintained their direction but were susceptible to magnetic deflection. Roentgen justified calling the new phenomena rays because of the O, Röntgen, then the news is true, And not a trick of idle rumour, That bids us each beware of you, And of your grim and graveyard humour. We do not want, like Dr. Swift, To take our flesh off and to pose in Our bones, or show each little rift And joint for you to poke your nose in. We only crave to contemplate Each other’s usual full-dress photo; Your worse than “altogether” state Of portraiture we bar in toto! The fondest swain would scarcely prize A picture of his lady’s framework; To gaze on this with yearning eyes Would probably be voted tame work! No, keep them for your epitaph, these tombstone-souvenirs unpleasant; Or go away and photograph Mahatmas, spooks, and Mrs. B-s-nt! —Punch, January 25, 1896 shadowy pictures they produce: bones in a hand, a wire wrapped around a bobbin, weights in a box, a compass card and needle hidden away in a metal case, the inhomogeneity of a metal. The ability of the new rays to produce photographs gave them great popular appeal and brought Roentgen fame. Many articles appeared in photography journals, and The New-York Times indexed the new discovery under photography. Since the rays exposed photographic plate, the public assumed they were some form of light. The physicist Roentgen concurred. Accepting Lenard’s claim that cathode rays were vibrations of the ether, Roentgen compared the new rays to them and forwarded the opinion that the two were ethereal, although different from visible, infra-red and ultra-violet light in that they did not reflect or refract. He suggested that cathode rays and X rays were longitudinal vibrations of the ether rather than transverse ones. Now that their existence was established, it was easy enough to experiment with the new X rays. Roentgen himself published only three papers on the subject, but others jumped quickly into the field. And not just physicists. Thomas Edison used modified incandescent light bulbs to produce the new rays. He boasted to reporters that anyone could make photographs of skeleton hands; that was mere child’s play. Within a month of Roentgen’s announcement doctors were using the X rays to locate bullets in human flesh and photograph broken bones. Dr. Henry W. Cattell, Demonstrator of Morbid Anatomy at the University of Pennsylvania, confirmed their Heinrich Rudolf Hertz, 1857–1894. (Courtesy of Deutsches Museum and AIP Emilio Segrè Visual Archives)