44 



Mr. J. W. Swan. Stress and other Effects 



on a metal disc in contact with one of the wires from the spark-gap 

 of the induction coil, the other wire, ending in a point, disc, or ball, 

 overhanging the surface at the distance of a few millimetres as in the 

 experiments with the viscous material described. On the production 

 of a single spark at the spark-gap by means of the trigger-action 

 mercury contact-breaker, a charge is conveyed to the resin. The 

 peculiar distribution of this charge, and that it is attended by strong 

 and enduring mechanical stress, can be made manifest either immedi- 

 ately, or many hours afterwards, by slightly warming the surface of 

 the resin. The result is a deeply impressed figure, having the same 

 character as the figure produced on the viscid dielectric. These 

 solid figures, if carefully developed, show much fine detail ; unfortu- 

 nately, this is not capable of complete illustration by photographs. 

 Difference of depth in the grooves is not fuJly indicated, neither is 

 there represented adequately a peculiar burring of the margin of the 

 grooves, especially in the negative figures, their edges rising slightly 

 above the plane surface, as though the resin had been finely carved. 



Persistency of the Stresses. — The persistency and fixity of the 

 electrification of the resin surface, determining the form and 

 character of the eventual figure, are very remarkable. If the 

 development of the effect of the charge is delayed for twenty-four 

 hours there is but little difference in the result from that which 

 follows immediate development. 



Bust Figures and Stress Figures combined. — It was found that the 

 heat-developed figures attracted dust from the atmosphere, and thus 

 formed, accidentally, a combination of a stress figure and a dust 

 figure. The attracted dust gave clear indications of electrification 

 beyond the limit of the stress figure, and brought out features of 

 detail which helped to explain the nature of the electrification. 

 Analysis of the character of the figures in this respect is still further 

 helped by combining a modification of the dust process of Lichtenberg 

 with the stress effect described in this paper. The modification 

 referred to consists in allowing the dust — the mixture of red lead 

 and sulphur proposed by Lichtenberg — to be drawn up to the elec- 

 trified surface from a cloud of dust. This assists in the selective 

 appropriation of the two substances, giving a redder colour on the 

 negatively electrified portions of the figure, and a yellower colour on 

 the positive portions ; and showing what the stress figure alone does 

 not show with equal clearness, how inseparable are the + and — 

 actions. The best effect is obtained by applying the dust process 

 before development by heat. Figs. 4 and 5 (Plate 1) represent 

 characteristic forms obtained by positive and negative discharges 

 with a metal ball electrode above the surface and a metal disc below, 

 fig. 4 being + and fig. 5 — . Figs 6 and 7 are corresponding figures 

 obtained with discs above and below, fig. 6 being +• and fig. 7 — . 



