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.

99. Elster and Geitel’s Experiment. Effect of Polarized Light upon the Cathode. Berlin Akad. ’95. Nature, Lon., March 28, ’95, p. 514. Proc. Brit. Asso., Aug. 16, ’94; Aug. 23, ’94, p. 406.—The X-rays have properties similar to those of light, and have their source in electricity. Quincke discovered that light which has been polarized perpendicularly to the plane of incidence is greatly increased as to its power of penetrating metals. Elster and Geitel used the following apparatus to determine the relation between polarized light and electricity. The current varied according to the angle of incidence and the plane of polarization. The apparatus comprised the following elements: An exhausted bulb, provided with a platinum anode, and a cathode consisting of potassium and sodium, combined in the form of a liquid alloy having a bright surface of reflection. The source of light was an oxyhydrogen flame, which played upon zircon instead of lime; a lens changed the diverging rays to parallel rays, which were polarized by a Nichol prism and allowed to fall upon the cathode. The electrodes of the vacuum bulb were connected to the poles of a generator of a current of about 400 volts. “The strength, of the current was greatest when the plane of polarization was perpendicular to the plane of incidence—i.e., when the electric displacements constituting light, took place in the plane of incidence, and when the angle of incidence was about 60°, i.e., the polarizing angle of the alloy itself.” Prof. Sylvanus P. Thompson confirmed these results by experiment. The rate of discharge was greatest, he said, when the plane of polarization was such that the Fresnellian vibration “chopped into” the surface. Polarized light, he reminded them, produced similar results upon selenium.

Although the domain of this book is necessarily limited to the consideration of phenomena connected with the internal and external energy of a discharge tube, yet if any other one subject is of special interest and utility in connection with the consideration of X-rays, it is that concerning the relation between the electric discharge and light, which has been thoroughly studied only during the past few years, and the accounts of the researches recorded in various periodicals and academy papers. Those readers, however, who desire to study this exceedingly interesting and novel branch of science, which in connection with the action of the internal cathode rays and X-rays upon electrified bodies, tends to uphold Maxwell’s theory as developed by mathematics and based upon early known facts and predicted discoveries, may find volumes upon this subject by referring to the citations below, named by Mr. N. D. C. Hodges and obtained by him by a search in the archives of the Astor Library. Of especial interest are those of Branly, § 99I, 99J, 99Q, 99S, 99T. Some notion as to the contents of the citations are given here and there.

99A. Koch’s Experiment. The Loss of Electricity From a Glowing Electrified Body. Wied. Ann., XXXIII., p. 454, ’88.

99B. Schuster and Anpenius’ Experiment. The Influence of Light on Electrostatically Charged Bodies. Proc. R. So., Lon., LXII., p. 371, ’87; Proc. Swedish Acad., LXIV., p. 405, ’87.—Many recent periodicals have set forth that ultra-violet light will discharge only negatively charged bodies. While this is practically or sometimes the case, yet these experimenters found that a positive charge was dissipated very slowly. They confirmed the results that the ultra-violet rays played the principle part in the removal of a negative charge. Polishing the surface accelerated the action. § 99, near beginning.

99C. Righi’s Experiment. Some New Electric Phenomena Produced by Light. Note 2-4, Rend. R. Acad. die Lincei, May 6, 20, and June 3, ’88.

99D. Righi’s Experiment. Some New Electric Phenomena Produced by Illumination. Rend. R. Acad. die Lincei. VI., p. 135, 187, ’88.—Confirmation of the results of other physicists, and a quantitative measurement determining that the E. M. F. between copper and selenium was increased 25 per cent. by illumination by an arc light. The selenium was in the form of crystals mounted upon a metal plate.

99E. Stolstow’s Experiment. Actino-Current Through Air. C. R., CVI., pp. 1593 to 95, ’88.—Liquids tested. Greatest absorbents of active rays most quickly discharged.

99F. Righi and Stolstow’s Experiments. Kind of Electric Current Produced by Ultra-Violet Rays. C. R., CVI, pp. 1149 to 52, ’88.—The discharge was accelerated by using a chemically clean surface. The burning of metals, for example, aluminum, zinc or lead in the arc light increased the discharging power.

99G. Bichat & Blondot’s Experiment. Action of Ultra-violet Rays on the Passage of electricity of Low Tension through Air. Comptes Rendus. CVI, pp. 1,349 to 51. ’88.—They employed arc lamps whose carbons had aluminum cores.

99H. Nacarri’s Experiment. The Dissipation of Electricity through the Action of Phosphorous and the Electric Spark. Atti di Torino. XXV, pp. 252 to 257. ’90.—The loss of charge was eighteen times less rapid in the dark through the air in a bottle, than when a piece of luminous phosphorous was placed in the bottle. The introduction of turpentine, which checked the glowing of the phosphorous, retarded the loss of charge.

99I. Branly’s Experiment. Photo-electric Current Between the two Plates of a Condenser. C. R. CX, pp. 898 to 901. ’91.—A positive charge was dissipated, and by a peculiar arrangement of the plates, screens, etc., and with particular materials, he was able to show that the rates of loss of a positive and negative charge were about equal. Numerous tests were instituted. If he is not mistaken, how closely related are X-rays and light. § 90. Those who wish to more thoroughly investigate this matter and verify the same, should study these experiments more in detail in connection with Schuster’s and Anpenius’ experiments (§ 99B), whose arrangement of the plates was the same as those of Branly.

99J. Branly’s Experiment. Loss of Both Electricities by Illumination with Rays of Great Refrangibility. C. R. CX, pp. 751 to 754. ’90.

99K. Righi’s Experiment. Electric Phenomena Produced by Illumination. Luer’s Rep. XXV, pp. 380 to 382. ’89.

99L. Borgmann. Actino-electric Phenomena. C. R. CVIII, p. 733. ’89. Jour. d. Russ. Phys. Chan. Ges. (2) XXI, pp. 23 to 26. ’89.—The photo-electric effect not instantaneous. A telephone served in the place of the galvanometer to detect the discharge.

99M. Stolstow’s Experiment. Actino-electric Investigations. Jour. d. Russ. Phys. Chan. Ges. (7-8) XXI, pp. 159 to 207.—It is necessary that the rays of light should be absorbed by the charged surface before having the discharging influence. § 99E. All metals are subject to the action, and also the aniline dyes. Two plates between which there is a contact difference of potential generate a current so long as the negative plate is illuminated. The effect is increased with the increase of temperature and is only found in gases, and is therefore of the nature of convection. He determined these principles by continuous work for two years. It should be remembered that in all these researches, the arc light is preferable, because the ultra-violet spectrum is six times as long as that given by the sun.

99N. Mebius’ Experiment. An Electric Spark and a Small Flame Employed. Bihang till K. Svenska Vet.-Akad. Hand. 15, Afd. 1, No. 4, p. 30, ’89.

99O. Worthington’s Experiment. Discharge of Electrification by Flames. Brit. Asso. Rep., ’90, p. 225.

99P. Fleming’s Experiment. Discharge Between Electrodes at Different Temperatures in Air and in High Vacua. § 99M, near end. Proc. Ro. So., LXVII., p. 118.

99Q. Branly’s Experiment. Hallwach and Stolstow’s Experiment. Loss of Electric Charge. Lum. Elect., LXI., pp. 143 to 144, ’91.—Branly obtained quantitative results. Hallwach found with the use of the arc light, a very small loss of positive electricity at high potentials; Stolstow, no such loss at potentials under 200 volts. Branly, with a 50 element battery and an arc light as the source of illumination, caused a discharge and thereby a constant deflection of 124 degrees of the galvanometer needle. The action of the light upon a positive disk caused a deflection of only three degrees by the same battery. With aluminum in the electrodes, the deflections were about 1400 and 24 respectively. Is it not sufficiently fully established that ultra-violet light will discharge not only negative but positive electricity? He experimented with substances heated to glowing or incandescence. Glass lamp chimneys at a dull, red heat, when covered with aluminum, oxide of bismuth, or lead oxides, withdraw positive charges. In the same way, for example, behaves a nickel tube in place of the lamp chimney.

99R. Wanka’s Experiment. A New Discharge Experiment. Abk. d. Deuts. Math. Ges. in Rrag., ’92, pp. 57 to 63.—He confirms the principle that the ultra-violet rays are the most powerful. A glass plate, which, as well known, cuts off most of the ultra-violet rays, was properly interposed and then removed and the difference noted.

99S. Branly’s Experiment. Discharge of both Positive and Negative Electricity by Ultra-Violet Rays. C. R., CXIV., pp. 68 to 70, ’92.—He further proves that ultra-violet rays of light will dissipate a positive charge. The experiments in this connection seem to prove more and more that the discharging power is only a matter of sufficiently high refrangibility of the rays of light.

99T. Branly’s Experiment. Loss of Electric Charge in Diffuse Light and in the Dark. C. R., CXVI., pp. 741 to 744. ’93.—A polished aluminum sheet was attached to the terminal of an electroscope properly surrounded by a metal screen. After a few days, the plate acted like any other metal plate polished or unpolished; it lost its charge very slowly, positive or negative alike, independently of the illumination. If it is then again polished, as for example, with emery paper and turpentine, it loses its charge rapidly in diffused light, which has passed through a pane of window glass, for example. Therefore, the ultra-violet rays are not alone effective, although most effective. The longer the time elapsing, after polishing, the slower the discharge takes place. Zinc behaved likewise, only more slowly. Other metals were tried. Bismuth acted differently from most metals. Whether charged positively or negatively, they exhibited rapid loss in the dark, in dry air under a metal bell, independently of the state of the polish.