470 REPORT— 1880. 



with it. The immersed plates become each coated with a layer of a darkish appear- 

 ance (eoside of silver ?), and this layer will give a current in the same direction as 

 that given by a silver plate coated in the old way with any emulsion of eosine. 



Platinum plates always give in these experiments much smaller resvdts than 

 silver plates ; hut this fact may he due to the circumstance that, vrith the substances 

 employed, silver may be a better vehicle for the transference of the energy than 

 platinum — apart from the consideration of chemical action. 



Several other substances, such as fluorescine, fuchsine, sidphate of quinine, &c. 

 were used, the results being less marked. 



Phosphorescence was studied by coating a platinum plate with a mixture of 

 gelatine and sulphide of calcium, and currents were produced by the action of 

 magnesium light. 



Experiments of the class last referred to are in progress. 



6. Electric Govvection-Currenfs. Bi/ Silvanus P. Thompson, D.Sc, B.A.^ 

 Professor of Experimental Physics in University College, Bristol. 



In a paper ' On the Action of Magnets on Mobile Conductors of Currents,' read 

 before this Section a year ago, the author discussed a number of cases of the flow 

 of electricity across a magnetic field. These included cases of true metallic con- 

 duction, of electrolj'tic conduction, and of those less-understood kinds of conduc- 

 tivity which occur in the voltaic arc, in the discharges in rarefied media, and in 

 the luminous brush-charge at a point. For the case of convection of electricity, 

 either automatically, by self-repidsion between electiified particles of a gas, or 

 mechanically, the electro-magnetic efiect is identical with that of a current in 

 which the same quantity of electricity would be transferred in the same time ; the 



' rate of convection ' -^ being in these cases the equivalent of ' the strength of the 



current.' 



Maxwell's theory (vol. ii. art. 768) concerning the virtual identity of a current 

 sheet and of an electrified sheet mo^•ing m its own plane -tt-ith a velocity equal to 

 * v,' may be extended to the case of linear currents. The identity may be 

 generahsed to all cases of convection-currents. 



Last year the author predicted that the brush discharge at a point would 

 experience a spiral twist when takmg place in the magnetic field. He has since 

 found this to be experimentally the case. 



The author also pointed out the similarity between the magnetic distortion 

 found by Reitlinger and Wachter ia electric ring-figures, and that found by himself 

 to be produced by the presence of a magnet on Nobili's figures. He also referi-ed to 

 Maxwell's theory as explaining some of the phenomena observed in the exhausted 

 tubes of Mr. Crookes, in which the discharges from the negative electrode behave 

 like convection-currents having a velocity less than the velocity of light. 



7. On a peculiar behaviour of Copper. By "William Henet Preece. 



From some experiments made in Dr. Warren De La Rue's laboratory it appeared 

 that in some cases copper wires did not acquire their normal resistance imtU currents 

 of electricitj' had passed through them. In several instances the resistance of 

 virgin copper was far higher than it was after electricity had passed through. 



8. On the proper form of Lightning Conductors. By William Heney Peeece. 



The question of the relative value of surface and sectional area in lightning- 

 conductors never ha\'ing been satisfactoiily solved experimentally, the authoi", with 

 the aid of Dr. De La Rue's gigantic battery, endeavomred to do so. He obtained 

 wire tubes and ribbons of copper and lead of similar lengths and weight, and 



