Roentgen Rays and Phenomena of the Anode and Cathode.

by Edward P. Thompson

From Sciagraph of Foot Deformed by Pointed Shoes. [§ 204].
By Prof. Miller.

From Hammer’s Molecular Sciagraph. [§ 117]., p. [114].

46. Thomson’s Experiment. Velocity of Striae Checked at the Cathode. Nature, Lon. Jan. 31, ’96, p. 330.—A tube 50 ft. long was exhausted, [§ 8a]., as to striking distance. In this particular experiment, he caused a single interruption in the primary of the induction coil, and observed the motion of the striae from the anode to the cathode by means of a rotating mirror. [§ 43]. The luminosity began at the anode and travelled toward the negative with a high velocity, but upon its arrival at the negative pole its velocity was checked. He said that the striae did not disappear at the cathode like a rabbit would in entering a hole, but they lingered around the electrode for some time. As a consequence of this delay, he found as expected, an accumulation of positive electricity, [§ 4], in the neighborhood of the cathode. It is a general principle, therefore, that when a discharge passes between a gas and metal, there is an accumulation, illustrating that the discharge experiences a difficulty or resistance. [§ 32] and [33]. The experimenter, Prof. J. J. Thomson, acknowledged that Profs. Liveing and Davy had noticed similar effects.

47. Thomson’s Experiment. Disruptive Discharge and Electrolysis. Nature, Lon. Jan. 31, ’95. Lect. S. Inst. The Electr., Lon. vol. 31, p. 291, 316, and vol. 35, p. 578. Trans. R. So., ’95.—The discharge of electricity through conducting liquids is, with scarcely an exception, (example, mercury) accompanied by a chemical action. Faraday and Davy both performed early experiments in this direction. Prof. J. J. Thomson has set forth some instructive facts and which act as evidence that there is a close relation between the disruptive discharge and chemical action between the dielectric and electrodes. [§ 6] and [7]. He made this experiment in connection with his investigations relating to the difficulty the positive electricity experiences in passing from a gas to the negative electrode. [§ 46]. He carried this experiment further, by testing gases of different chemical natures. The apparatus he employed consisted first of an alternating current generator, a high tension converter, a bulb for containing first one gas and then another, whose metal electrodes were connected with the secondary of the transformer, and an electrometer connected to a third electrode which could be moved about within the bulb. The operation was as follows: when the bulb contained oxygen which is an electro-negative gas, the third movable electrode received a positive charge in whatever part of the bulb it was moved to, but with hydrogen instead of oxygen at atmospheric pressure, the third electrode received a positive charge far away from the arc between the other electrodes, but very near the arc it received a negative charge. He then rarefied the atmosphere of hydrogen and he noticed that the space where the third electrode became negative, contracted, and at about 1/3 of an atmosphere became practically nothing, so that the said third electrode connected to the electrometer became slightly positive at all points within the hydrogen. [§ 4]. The next step consisted in using a bulb, having oxydized copper electrodes and a hydrogen atmosphere at the pressure where there was only positive electricity, that is about 1/3 of an atmosphere. This remarkable phenomenon occurred; there was no positive electricity, but only negative. When the copper oxide was reduced, the positive electricity only, existed in all parts of the bulb. In brief, bright copper electrodes left a positive charge in the gas, while oxydized electrodes left a negative charge. He argued upon the results of this experiment to account for the delay in the passage of the electricity from the gas to the metal, [§ 46]. In later experiments, he used the spectroscope to detect decomposition. [§ 6], at end.

48. De La Rue and Müller’s Experiment. Heat Striae. Phil. Trans., vol. 159, 1878—They arranged for the best conditions, that is, when a small number of striae occurred in conjunction with a wide, dark interval. [§ 44]. They found that the heat was greatest at the position of maximum luminosity, but they also found that heat was generated at the dark spaces. A novel feature was the discovery of the development of heat in the middle of the tube even when there was no luminosity, [§ 9a], near end, so that they thought it probable there may be what might be termed heat striae, independently of luminous striae.

49. Spottiswoode and Moulton’s Experiment. Sensitive State. Air-Gap in Circuit Forms Best Method of Obtaining. Branch Current to Earth Verified by a Telephone. Sensitive State by a Single Quick Discharge. Phil. Trans., 1879, p. 165, and April 8, 1880.—By sensitive state of luminous effects in a Geissler tube is meant the susceptibility of the light ([§ 28]) to an outside conductor connected to earth. Fig. 5, p. [17]. When one’s hand is brought near a Geissler tube the change near the hand sometimes occurs and sometimes it does not. [§ 8]. In the first place, the effect is more easily noticeable if the vacuum tube is comparatively wide or thick in diameter. With the electric egg, for example, the luminous effect, instead of extending more or less across the space between the electrodes, reaches from one of the poles to a conductor on the outside of the egg, provided said conductor has an earth connection or large capacity. Some of the light continues to exist nevertheless between the two poles. The general principle is that the division exists because of the re-distribution or branching of the disruptive discharge. It was not known why the luminosity should be affected by such an outside conductor sometimes, and remain the same at other times but the above named experimenters discovered causes which could be depended upon to produce the sensitive state. The apparatus will be described. They had the usual Geissler tube with the platinum wire electrodes, and a Holtz machine as the generator. They were led to believe that intermissions of the current had a great deal to do with the production of the sensitive state, and accordingly they arranged for an air-gap in circuit with the machine and with the vacuum tube. [§ 51]. They not only observed that such a gap caused the sensitive state, but that an increase in the length of the gap made the luminous column more sensitive. They increased the gap so much that the ramifications of the light could be seen. If an induction coil is employed as the secondary generator, a condenser should be coupled up in connection with it. The two in combination thereby produce the sensitive state, but upon cutting out the coil and charging the tubes from the condenser the sensitiveness can not be detected. Instead of the permanent air-gap, may be employed a rapid circuit interrupter, coupled up between a Holtz machine and a vacuum tube. The manner of coupling up is to place the interrupter in a shunt to the vacuum tube. Difficulty had been found in early experiments to obtain the sensitive state with those vacua which give striae. With a rapid circuit interrupter and an induction coil, the breaks occurring 240 per second, the luminous column was not only broken up into striae, but were acted upon by the approach of an outside conductor connected to earth. The sensitive state is not always made apparent by the appearance of attraction of the luminous light to the outside conductor. Sometimes the light seems to be repelled. These two phenomena may be caused in the same tube. This feature of the sensitive state constitutes the beginning of radiations of energy through the walls of a vacuum bulb, like X-rays. Some action or other in these cases takes place through the glass. They tried an experiment in which one of the electrodes of the vacuum tube was entirely on the outside. The electrical discharge was found to be sensitive, for the discharge was changed in its appearances by the presence of an outside conductor connected to earth. Another cause of the sensitive state was observed, namely, the brevity of the charge. This may be illustrated with a Leyden jar, which is known to give an almost practically instantaneous discharge. A single discharge from such a jar produced a flash of light which was in the sensitive state. The nomenclature by which the experimenters defined the cause of the phenomena is made up of the words: Re-distribution of electricity, and a relief of the external strain.

49a. No re-distribution took place unless the outside conductor was connected to earth or to a conductor of large capacity, nor would an outside conductor, which was already charged, serve to exhibit the sensitive state. The re-distribution effect was proved by means of a telephone connected in circuit between the outside conductor and the earth Fig. 5, p. [17]. When the state was sensitive, that is, during the use of the air-gap, the telephone produced a sound in unison with the intermissions occurring at the air-gap. [§ 9] and [9a].

50. Reitlinger and Urbanitzky’s Experiment. Sensitive State Illustrated by a Flexible Conductor Within the Discharge Tube. Proc. Vienna Acad., 1879. Nature, Nov. 20, 1879.—The discharge tube was 20 cm. long. It had the usual platinum electrodes, and it stood upright. From the upper electrode, was suspended a strip of tinfoil in the middle of the tube, which was connected to a pump so that the density of the gas could be varied. At atmospheric pressure, the secondary current of a Ruhmkorff coil connected to the electrodes caused the strip to be attracted to the glass tube. The attraction was less and less as the process of exhaustion was carried on, and when a pressure indicated by 7 mm. was reached, the strip was neither attracted nor repelled, but hung downward the same as without any electricity whatever, but it was attracted by a neighboring shell-lac rod which had been rubbed with cloth, and it was repelled by a glass rod which had been rubbed with amalgam, it being assumed that the strip was connected to the anode. § [36]. The opposite action took place when it was connected to the cathode. As the exhaustion continued and became greater and greater, these actions died away also up to a rarefaction of about 4 mm. Independently of the degree of rarefaction, the flexible strip of tinfoil was always deflected by an outside conductor connected to earth. [§ 49].

51. Tesla’s Experiment. Incandescent Electrode by High Potential and Enormous Frequency. System Referred to by Roentgen for Generating Powerful X-Rays. U. S. Letters Pat., No. 454, 622, June 23, ’91. Martin’s Researches of Tesla; Trans. Amer. Inst. Elec. Engineers, May 20, ’91; Elec. Review, N.Y., June 24, ’93, p. 226; Lect. Franklin Inst., Feb. 24, ’93, and Nat. Elec. Light Asso., Mar. 1, ’93; also Lect. in Europe. Later he again experimented in this direction, see Elec. Review, N.Y., May 20, ’96, p. 263.—By the U. S. Patent Office he was granted, among other claims, the following: “The improvement in the art of electric lighting herein described, which consists in generating and producing for the operation of lighting devices, currents of enormous frequency and excessively high potential, substantially as herein described.” A simple combination of circuits together with great skill in the construction of apparatus involving high powers of insulation, resulted in the production, within a vacuum, of an electrode radiating intensely white light. The circuit may be easily traced in the diagram Fig. 17 p. [17]. Briefly described, there may be noticed an alternating current generator of comparatively low E. M. F. The current from this generates a secondary current by means of an induction coil. This secondary current generates a tertiary current by a second induction coil. An air-gap for automatic and intermittent disruptive discharges, [§ 49] near end, is in the circuit of the secondary coil of the first named induction coil, which is directly charged by the alternating current generator. The gap may be noticed between the two balls. In shunt to the air-gap is a condenser (see Fizeau, chapter I.) represented by several parallel lines. The lamp consists merely of an evacuated bulb having an electrode of carbon or other refractory material, which is connected to one pole of the last secondary coil while the other pole may be outside, and may consist, for example, of the walls of a room, which in such a case should be of some electric conducting material. The higher the vacuum the more intense the light; he found no limit to this rule. Fig. 16a p. [17] illustrates his ideal method of lighting a room. He found that with two plates at a distance apart as indicated and connected to the poles of the coil, and with electrodeless vacuum bulbs, the latter became bright in space—no mechanical or electrical connection other than air and the assumed ether.