produced in Resin* by Electrification. 



45 



Figs. 8 and 9 are additional examples of negative discharge figures 

 with larger disc electrodes. 



An excellent dust figure, in which the result of + electrification is 

 strongly developed, is obtained by suspending, face downwards, an 

 electrified resin surface in a very thin fume-cloud produced by burn- 

 ing magnesium ribbon. The fume should be enclosed in a box, or 

 under a glass shade, and an hour should elapse for the coarser parti- 

 cles to subside before the introduction of the electrified surface. 

 Sulphur in a state of sublimation can also be used in the same way 

 with good effect, especially for very small areas of electrification, 

 where microscopically fine development is required. On the whole, 

 however, I have found nothing better than red lead and sulphur 

 ground separately to very fine powder, and used very dry in a dust- 

 ing box, the electrified surface always being downward when exposed 

 to it. 



With the object of finding the degree and kind of interaction 

 between the positive and negative electrification produced on opposite 

 sides of a solid dielectric, interposed in the path of a single discharge, 

 the following experiment was made : — A thin plate of glass was 

 coated on both sides by dipping in melted resin, this was electrified 

 by bringing the secondary terminals of the induction coil, arranged 

 as in the experiments already described, to opposite sides of the plate. 

 The terminals were brass balls 8 mm. diameter, placed in a vertical 

 line, the + above, the — below the plate in a horizontal position ; 

 the + ball was 1 mm. distant from the upper surface of the plate, 

 and the — ball was in contact with the under surface. Under these 

 conditions when an 8-mm. spark passed at the spark-gap, reciprocal 

 figures of a very interesting character were produced, a + figure on 

 the upper surface and a — figure on the under surface. 



To enable photographs to be taken of these figures without in- 

 terference, the experiment was repeated with the variation that a 

 plate of ruby glass coated with resin on both sides was used instead of 

 clear glass. The latent figure was first developed by means of the red 

 lead and sulphur cloud, and afterwards the stress effect was brought 

 out by heat. Fig. 10 shows the form of the figure on the + side, and 

 fig. 11 that on the — side. When these double figures are viewed by 

 transmitted light, it is seen that the interior — rays on one side, 

 coincide with the inner ends of the outward, streaming 4- rays on 

 the opposite side. 



That the depth of penetration of the charge w r hich produces these 

 figures is very small is shown by the almost complete discharge 

 effected by washing the electrified surface with water. 



The experiments seem to show that when an electric discharge 

 takes place through air, its propagation is attended by a structural 

 arrangement of the air brought under the influence of the discharge, 



