produced in Resin, Sfc, by Electrification. 39 



It was found that clear Bordeaux resin in a viscid state (viscidity 

 being brought about either by heat or by the addition of resin oil) is 

 responsive to the mechanical stress consequent on electrification by 

 non-luminous discharges ; and if it is so acted upon while in the 

 solid state, and afterwards superficially softened by heat, there 

 results a new kind of electric discharge figure, analogous to the dust 

 figures of Lichtenberg and Lord Armstrong, but showing some remark- 

 able peculiarities which throw additional light on the mechanism of air- 

 conveyed electrical discharges, and on the location and nature of the 

 stresses imparted to the dielectric. I ascertained that a smooth sur- 

 face of resin is retentive of an electric charge to an extraordinary degree, 

 that after more than two months the lines of an electric discharge 

 figure, as developed by heat, and as further developed by the acci- 

 dental attraction of atmospheric dust to the electrified parts of the 

 surface, were still attractive of dust in a discriminative manner, no 

 change being observable upon re-dusting either in the arrange- 

 ment or definition of the lines of electrification as originally 

 developed. 



The apparatus employed consisted of an induction coil or a Wims- 

 hurst machine, and a supporting stand, the rod of which carried two 

 clips and a stage, the supporting part of the stage being made of 

 strips of thick plate-glass, and the rest of wood. The clips held 

 conducting wires, which passed through bent glass tubes, and went 

 to the secondary terminals of the induction coil, or to the conductors 

 of the Wimshurst machine ; the discharging arms in either case con- 

 stituted an adjustable spark-gap in parallel with the wires ending 

 above and below the stage. The stage terminals were balls, discs, or 

 points; the pairs employed in different experiments varied in size and 

 form, and the pair used together were sometimes dissimilar. The 

 resin was the colour of amber ; in some of the experiments it was 

 used in a solid state, but fused to the form required for experiment ; 

 in other experiments it was softened to semi-liquidity by the addition 

 of 20 per cent, or more of resin oil, the mixture being made by fusion 

 together of the resin and oil. The compound with 20 per cent, of 

 oil has the consistency of treacle at a temperature of 20° C. ; at 

 12° C. it is nearly solid, yet plastic enough to yield to the mechanical 

 stress- action generated by the projection upon its surface of an 

 electric discharge of the kind employed in the experiments. At the 

 higher temperature the viscid liquid is well suited for showing the 

 great disturbance produced by repeated discharges, and when at the 

 lower temperature it is convenient for observing the more persistent 

 forms of the figures produced on the surface by single discharges 

 under various conditions. When it was required that the stress 

 figures should be permanent, resin either alone or with not more than 

 2 or 3 per cent, of resin oil was used. 



