95. Prof. J. J. Thomson showed how it was possible that “longitudinal waves can exist in a medium containing moving charged ions, and in any medium, provided the wave length is so small as to be compared with molecular dimensions, and provided the ether in the medium is in motion. It follows from the equation of the electro-magnetic field that the ether is set in motion in a varying electric field. These short waves would not be refracted, but in this respect they do not differ from transverse waves, which on the electro-magnetic theory would not be refracted if the wave length were comparable with molecular distances.” From Elect. Eng., N.Y., Mar. 18, ’96, p. 286, in reference to a paper before the Cam. Phil. So.
96. One of the very first questions asked in reference to a discovery is as to its practical utility. Already, we have important applications in one of the most humane directions, and that is in connection with diagnosis. Sciagraphs can also be employed in schools for the purpose of education, in some departments of anatomy, etc. The interest that it excites and the amusement that it affords are not to be overlooked, for anything in the nature of recreation possesses utility. However, we may greatly thank all experimenters who have investigated the subject, and who have not left its development alone to Roentgen; for predictions as to the utility of a discovery, however, apparently exaggerated, are very often proved, by subsequent developments, to have been underrated. Upon this point Prof. Boltzmann, in Zeit. Elect., Jan. 15, ’96, see also, The Elec., Lon., Jan. 31, ’96, p. 447, stated, “If we remember to what discoveries the most insignificant new natural phenomenon, such as the attraction of small objects by rubbed amber, of iron by the lode-stone, the convulsive twitches of a frog’s leg due to electric discharges, the influence of the electric current upon the magnetic needle, electro-magnetic induction etc., has led us, one can imagine to what applications an agent will be turned, which a few weeks after its discovery has given rise to such surprising results.”
97. Soon after hearing, (about the first of Feb. ’96,) of the Roentgen discovery, it occurred to the author to carry on experiments with fluorescence, but finding that it was inconvenient to work in a perfectly dark room, and, recognizing that black cardboard had practically no effect upon absorbing the X-rays, he devised a sciascope (daily papers, Feb. 13, and Elect. Eng., Feb. 19) which he afterwards learned was independently invented and used at about the same time by Prof. William F. Magie, of Princeton University, (see Amer. Jour. Med. Sci., Feb. 7, ’96 and Feb. 15, ’96) and by Prof. E. Salvioni, of Italy under the name of cryptoscope, (see Med. Sur. Acad. of Perugia, Italy, Feb. 8, ’96.) In about a month afterwards (Elect. Eng., N.Y., Apr. 1, ’96, p. 340) the instrument (with phosphorescent calcic tungstate [§ 13]. in place of fluorescent barium platino cyanide) was again published under the name of the Edison fluoroscope. There are probably many other claimants—some professor in London—name forgotten. They all consist of a tapering tube with a sight hole at one end and a fluorescent screen in the other, which is closed by opaque card board. (Frontispiece at Chap. X). For the sake of conformity, the words sciagraph and sciagraphy and similar derivatives, and in view of the meaning of the radical definitions, have been employed throughout the book. The objection to the word fluoroscope is that the instrument is practically universally employed in seeing the shadows of objects, otherwise invisible to the naked eye, rather than to test fluorescence. The name sciascope was early suggested by Prof. Magie. For those who wish to make a screen, the author may state that he obtained a good one by mixing pulverized barium platino cyanide with varnish and spreading the mixture over a sheet of tracing cloth.
CHAPTER VIII
97a. Hertz’ Experiments. Electrified Bodies Discharged by Ultra-Violet Light of a Spark and by Other Sources of Light. Berlin Akad. II., p. 487, ’87. Wied Ann. XXXI, p. 983. English translation of the above. Lon. and N.Y. Macmillan, p. 63, ’93. From notes by Mr. N. D. C. Hodges.—This is the all-important initial work of H. Hertz. The source of light was a spark, and the great discovery resulted from a combination of circumstances and was unsought; but by studying and testing the matter, he found the cause. Two induction coils, a and b, having interrupter d, were included in the same circuit, as shown in the figure. The sparking of the active one (A) increased the length of the spark of the passive (B) [§ 10]. He sought the cause. The discharge was more marked as the distance between the sparks was reduced. Sparks between the knobs had the same effect as those between points; but the effect was best displayed when the spark B was between knobs. The relation between the two sparks was reciprocal. The discharging effect of the active spark (A) spread out on all sides, according to the laws of light, first suggesting that light was the cause. Most solid bodies acted as screens, s. Liquid and gases served more or less as screens. The intensity of the action increased by the rarefaction of the air around the passive spark, i.e., in a discharge tube. The radiations from the spark, A, reflected from most surfaces, according to the laws of light, and refracted according to the same laws, caused the discharge. The ultra-violet light of the spark A was inferred to be the active agent in producing the discharge. The same effect was produced by other sources of light than the electric spark. The conclusions were afterwards confirmed by many, and subordinate discoveries originated. [§ 98]—[99T].
97b. Wiedemann and Ebert’s Experiment. Light Discharges Cathode, but Has No Influence upon anode, Nor Air-Gap. Different Gases and Different Pressures. Wied. Ann. XXXIII, p. 241. 1888. From notes by N. D. C. Hodges.—The arc light was used in place of the active spark of Hertz. Principal result was that the effect depended on the illumination of the cathode ([§ 99].) The illumination of the anode or of the spark-gap did not influence the discharge. The very character of the charge was altered by the action of light upon the cathode. The influence of the illumination of the cathode did not consist solely at the starting of the spark, but lasted as long as the sparks continued to pass. With decreasing pressure of surrounding gas, the effect first increased ([§ 97a]) to a maximum, and then decreased ([§ 54]). The illumination had an effect on the path of the sparks, the path being perpendicular to the rays of light. The best results were obtained with carbonic acid gas. Hydrogen was next, and then air. They were contained in the tubes surrounding the poles. The character of the gas also had an influence on the rays which would produce the effect, with carbonic acid gas the effect showing itself even with the visible rays.
98. Elster and Geitel’s Experiment. Negatively Charged Bodies Discharged by Light. Wien. Berichte. Vol. CI, p. 703, ’92. Wied. Ann. Vols. XXXVIII, XXXIX, XLI, XLII, XLIII, XLIV, XLVI, XLVII, LII. Nature, Lon., Sept. 6, ’94, p. 451.—The elements employed for carrying on the experiment consisted of a delicate electroscope and certain metals, including aluminum, amalgamated zinc, magnesium, rubidium, potassium and sodium. Some of the experiments were made on the top of Mount Sonnblick, the same being 3,100 m. high, where the discharging power of light was found to be about twice as great as at Wolfenbuttel, which was at the level of 80 m. The whole time for the discharge was only a matter of a few seconds. The greater rapidity of discharge at the higher level was attributed to the greater proportion of ultra-violet rays (Hertz), which are the most easily absorbed by the atmosphere, according to Langley. All metals are not discharged alike by the action of light. The law follows the electro-positive series in such a way that the more electro-positive the metal, the longer the wave length of light necessary to produce the discharge. In experiments with potassium, sodium and rubidium, they made them successively, the cathode in a bulb of rarefied hydrogen. In this case it was found that the light of a candle, even at so great a distance as 7 m., would cause the discharge. Rubidium was sensitive in this respect to the red light from a heated rod of glass. Elster and Geitel were able also to discharge, by light, some non-metallic bodies, like calcic sulphide, when so prepared that it had the property of phosphorescing, and also darkly colored fluorites. Independently, the phenomenon is of importance, because Elster and Geitel determined that there was some common cause as to the discharge of bodies of light and the discharge from the earth’s surface. A series of experiments lasting three years, consisted in investigating the relation of the ultra-violet rays from the sun simultaneously to the quantity of charge in the atmosphere. The results acted as evidence of the explanation of the daily and annual variation of atmospheric potentials. These experiments are of importance in connection with X-rays, because Röntgen and Prof. J. J. Thomson subsequently, and possibly others independently, discovered that X-rays produce, not only a like, but a more extended action in that there is not so great a difference between their power to discharge negatively and positively electrified bodies. [§ 90a]. In the further developments of their ideas, they tried the action of diffused day-light upon a Geissler tube traversed by vibrations which were produced by a Hertz vibrator (see recent book on Hertzian waves), the tube having an electrode of metal of the alkaline group. They were able to adjust the combination so that the presence of a little day-light would initiate a luminous discharge, while in the dark such a charge ceased. [§ 14a].