32. Thomson’s Experiment. Resistance Offered to Striae by a Thin Diaphragm. Lect. Royal Inst. Nature, Lon. Jan. 31, ’95, p. 333.—It has often been remarked that lightning always takes the easiest path. The same has been noticed with references to the artificial electric spark. Prof. J. J. Thomson, F.R.S. performed an experiment, which not only confirms this principle but does so in an emphatic manner, and proves it true in reference to the electric discharge in rarefied gases. He arranged a very thin platinum diaphragm so as to divide a Geissler tube into two compartments, Fig. 19, p. [17]. He then formed a passage way around the diaphragm, which could be opened and closed by mercury, by respectively lowering and raising the lower vessel of mercury along the barometer tube. When the passage way is opened around the diaphragm, the luminosity extends through the passage way in preference to going through the diaphragm. When the passage way is closed by mercury, the discharge goes through the thin metal plate. The same was found to occur when the platinum leaf was replaced by a mica scale.
33. Sir David Solomon’s Experiment in 1894. Proc. Royal So., June 21, ’94. Nature, Lon. Sept. 13, ’94, p. 490.—With a tube having a perforated diaphragm, he noticed a “forcing effect” at and near the hole. The striae had the appearance of being pushed through from the longer part of the tube—the diaphragm not being in the centre. There was no passage way around the diaphragm—only through the small puncture. [§ 19].
CHAPTER III
34. Riess’s Experiment. Electric Images. Riess’s Reibungs. vol. 2, § 739.—He laid a coin upon a plate of glass and charged the same electrically about one-half of an hour or more. Upon removing the coin and sprinkling the plate with dust, an engraving of the coin was visible upon the glass. [§ 13]. A suitable dust is licopodium powder.
35. Sanford and McKay’s Experiment. Electrographs. Original Contribution by Prof. McKay of Packer Inst., Brooklyn, May, ’96.—The picture of the coins in Fig. [IX], was produced by the apparatus shown in Fig. [VIII], t, t, tinfoil, p, photographic plate with coins on sensitive side, all wrapped in black paper. Fig. [VIII] represents the general arrangement for taking electrographs. This particular one was made by removing the upper tinfoil and touching each coin successively with wire from one of the poles, while the other wire was connected with tinfoil on the opposite side. The condenser thus formed is charged and discharged many times by a Holtz machine or induction coil. This is not a new discovery, it was first described by Prof. Sanford, I think, of Leland Stanford University, two or three years ago. Other claimants of earlier date probably exist.
36. Lichtenberg’s Experiment. Dust Figures. Pictures Drawn with Anode and Cathode. Göttingen, 1778-79. Motum Fluidi Electriciti.—He drew two independent superposed pictures upon a flat surface of an insulating material, for example, rosin. One picture was drawn with one terminal of a charged Leyden jar. Another picture was drawn with the other terminal of a charged Leyden jar. He sprinkled upon the surface over the two pictures, a dust made of a mixture of red lead and sulphur powder. The former became attracted to the picture drawn with the cathode, and the latter to that made with the anode, so that the two figures were clearly visible. Before sprinkling the powders upon the surface it is necessary to stir them together whereby they become oppositely electrified.
