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Some Experiments prior to Lenard’s.

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. ssss1. 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.