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 199512 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 ex￾periments inspired Roentgen to won￾der if the rays in an attenuated form really traveled farther, and he planned experiments to see if a sensitive electroscope would mea￾sure 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 mer￾chant 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 doctor￾ate 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 Stras￾bourg. 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 un￾til the first dark space expanded to fill the entire tube and all glows dis￾appeared, 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 ef￾fects of static electric forces. As was to be characteristic of the new ray physics to come—the phys￾ics 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 pro￾gram 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