Roentgen Rays and Phenomena of the Anode and Cathode.

by Edward P. Thompson

From Magnetographs by Prof. McKay. p. [25].
1. Platinum wire.
2. Copper gauze.
3. Iron gauze.
4. Tinfoil.
5. Gold-foil.
6. Brass protractor.
7. Silver coin.
8. Platinum-foil.
9. Brass.
10. Lead-foil.
11. Aluminum.
12. Magnesium ribbon.
13. Copper objects.

From Sciagraph of Various Objects. p. [130].
By Prof. Terry, U. S. Naval Academy.

11. Glow by Discharge. Glow Changed to Spark. Motion of Air. Continuous Discharge During Glow. The glow was most easily obtained in rarefied air. The electrodes were of metal rods about .2 of an inch in diameter. He also obtained a glow in the open air by means of one or both of the small rods. He noticed some peculiarities of the glow. In the first place, it occurred in all gases and slightly in oil of turpentine. It was accompanied by a motion of the gas, either directly from the light or towards it. He was unable to analyze the glow into visible elementary intermittent discharges, nor could he obtain any evidence of such an intermittent action, [§ 8a]. No sound was produced even in open air. [§ 9]. He was able to change the brush into a glow by aiding the formation of a current of air at the extremity of the rod. He also changed the glow into a brush by a current of air, or by influencing the inductive action near the glow. The presentation of a sharp point assisted in sustaining or sometimes even in producing the glow; so also did rarefaction of the air. The condensation of the air, or the approach of a large surface tended to change the glow into a brush, and sometimes into a spark. Greasing the end of the wire caused the glow to change into a brush.

12. Lullin’s Experiment. Spark. Penetrating Power. Passage Through Solids. Encyclo. Brit. Article Electricity. He placed a piece of cardboard between two electrodes and discovered that a spark penetrated the material and left a hole with burnt edges. When the electrodes were not exactly opposite each other, the perforation occurred in the neighborhood of the negative pole. Later experiments have shown that a glass plate, 5 or 6 cm. in thickness, can be punctured by the spark of a large induction coil. The plate should be large enough to prevent the spark from going around the edges. The spark is inclined, also, to spread over the surface of the glass instead of piercing it, [§ 24]. Glass has been cracked by the spark in some experiments.

13. Fage’s Experiment. Spark. Penetrating Glass. Holes Close Together. Practical Application. La Nature, 1879. Nature, Dec. 26, 1879, p. 189. The length of the spark from the secondary coil in air was 12 cm. One terminal of the secondary passed through an ebonite plate (18 cm. × 12) and touched the glass. Olive oil was spread around said terminal ([§ 11] at end), and served to insulate the same. Oil dielectric in this connection originally employed at least prior to 1870. Remembered by Prof. Anthony as far back as 1872, who often performed the experiment according to instructions contained in a publication. The other terminal of the secondary coil was brought against the glass opposite the first terminal. The spark was then passed and the glass perforated, [§ 12]. By pushing the glass along to successive positions and passing the spark at each movement, holes could be made very close together. In Nature, of 1896, the author noticed that certain manufacturers were introducing glass perforated with invisible holes to be used for windows as a means of ventilation without strong draughts. Perhaps the fine holes were made by means of the electric spark.

14. Knochenhaurer’s Experiments. Conducting Power of Gas. Spark. Penetrating Power. Relation of E. M. F. to Pressure of Gas. 1834. Pogg. Ann., Vol. LVII., and Gordon, Vol. II. Boltzmann’s experiment (Pogg. Ann., CLV., ’75), and calculation indicated that a gas at ordinary pressure and temperature must have a specific resistance at least 10²⁶ times that of copper. Pogg. Ann., CLV., ’75. Sir William Thomson (Kelvin) confirmed this limit for steam, and Maxwell the same for mercury and sodium vapor, steam and air. From Maxwell’s MSS. Herwig was not sure but that the Bunsen burner flame and mercury vapor conducted. He allowed for the conductivity of the walls of the glass container. Braun treated of the conductivity of flames. Pogg. Ann., ’75.

14a. Varley found that 323 Daniel cells only just initiated a current through a hydrogen Geissler tube, and only 308 cells continued the current after once started. Knochenhaurer found that Harris’ (Phil. Trans., 1834) law did not hold exactly true, and that the ratio between the E. M. F. and the air pressure becomes greater and greater as the pressure becomes less and less. Harris thought the ratio was constant. The limits of his pressures were from 3 to 27.04 inches of mercury. Stated in other words, his results were the same as those of Harris and Masson (Ann. de Chimie, XXX., 3rd Se.), except that a small constant quantity should be added. [§ 16].

15. Gordon’s Experiment. Dust Particles Hasten Discharge. Gordon, Vol. II. Other experimenters had investigated the phenomena of the electric spark with different densities of the dielectric by a spark produced by a frictional or an influence machine, or, in a few cases, by powerful batteries without coils, while Gordon claims to be the first to carry out these experiments with an induction coil. He observed that when the discharging limit was nearly reached, small circumstances, such as a grain of dust or a rusting of the terminal by a former discharge, would cause the discharge to take place at a lower E. M. F., which should be allowed for.

16. Kelvin’s Experiment. Proc. R. So., 1860. Encyclo. Brit., Art. Elect. He used as the terminals, two plates. One of them was perfectly plane, while the other had a curvature of a very long radius. The object of this arrangement was to obtain a definite length of spark for each discharge. The plates were gradually moved away until the spark would no longer pass, and the reading of the distance was noted. The law which he found cannot well be expressed in the form of a rule or principle, because it is of a rather intricate nature, but a discovery resulted, namely in the case where the distance was greater, the dielectric strength was smaller for respective distances of .00254 and .535 cm. Many theoretical considerations in reference to this matter have been presented, notably that of Maxwell in his treatise on Electricity and Magnetism, Vol. I.