TRANSACTIONS OF SECTION A. 637 



been supposed to occur. This unexpected behaviour may be explained by the 

 proper physical theory, but cannot be accounted for by the ohmic law for magnetic 

 circuits, 



2. On M. E. Branlys Experiments on Electrical Resistance. 

 By Dawson Turner, M.I). 



The author had the pleasure, when in Paris, of seeing some of M. E. Branly's 

 experiments on the influence of currents of high potential upon electrical resistance 

 (an account of which has been published by M. Branly in the ' Comptes Rendus,' 

 November 24, 1890, and January 12, 1891 ; also in the 'Bulletin de la Soc. Int. 

 des Electriciens,' No. 78, May 1891). The author has repeated some of these. 

 It is well known that metallic powders present a very great electrical resistance. 

 This can be lowered to an extraordinary degree by the passage of a spark in 

 their neighbourhood. The author, following M. Branly, has tried, amongst 

 other substances, powdered aluminium, copper, annealed selenium, iron filings, 

 small shot, mixtures of aluminium and resin fused into a solid stick, &c. The 

 best results were obtained with the first two. A short glass tube is filled with 

 powdered aluminium, and placed, by means of copper wires passing through corks, 

 in circuit with one or two cells and a galvanometer. The resistance of the 

 aluminium is very great, and the needle of the galvanometer will remain at zero. 

 If, however, a spark from an induction coil be passed anywhere in the vicinity, the 

 resistance will become at once very much diminished and the galvanometer needle 

 will be deflected. Thus, though there was at first no appreciable current, there 

 may now be one of 250 m.a. The powder will, if undisturbed, remain a conductor 

 for some little time ; but if the table be shaken, or the glass tube be moved, the 

 initial resistance at once returns and the needle passes back to zero. After the 

 resistance has been once lowered in this way the powder gets into a very sensitive 

 condition ; a spark at a great distance lowers the resistance, and the slightest jar 

 restores it. The author was not successful with ordinary iron filings, perhaps 

 because they were not sufiiciently finely divided. Mere shaking does not restore 

 the resistance of the solid stick of aluminium and resin, but the application of a 

 little warmth is sufficient. 



The experiments would appear to suggest that the molecules of the powders 

 become arranged in such an order as to conduct electricity when an electrical 

 discharge occurs in their neighbourhood, and that the slightest mechanical jarring 

 suffices to destroy this arrangement. It does not appear to matter which way the 

 induction current is passed. 



3. On Electrical Discharges. 

 By Professor Eilh. Wiedemann and Dr. H. Ebert. 



The object of the experiments to be described was to study the phenomena ol 

 electrical discharges in rarefied gases under the influence of electrical oscillations of 

 a well-defined period, so as to get a deeper insight into their nature. For pro- 

 ducing regular electric motions the authors adopted the method of Lecher, but 

 tbey substituted for the induction coil, with its complicated form of discharge, a 

 Tcipler machine of twenty rotating plates, purchased out of a grant given by the 

 Elisabeth Thompson fund. The Lecher wires were between 6 and 14 metres long ; 

 bv means of bridges resonating circuits of definite period were formed ; the ends of 

 the wires were connected with the plates of a Kohlrausch condenser — the ' End 

 Condenser.' 



Like other experimenters, the authors found in the beginning a great difficulty, 

 sometimes impossibility, in getting a luminosity of the tube laid between or at the 

 sides of the end condenser, if it was of small diameter. Larger tubes were much 

 more easily excited. But it was found that tubes, not too small, with inside coatings 

 of platinum, always gave luminosity, and that if they were brought near other in- 

 sensitive tubes these also began to glow, and even did so after the exciting tube 

 had been removed. 



