produced in Resin, by Electrification. 



41 



circular, or nearly circular, band or ring, more or less indented in 

 outline, enclosing leaf -like rays which tend towards the centre. 

 These are relatively broader and less branching than the rays of the 

 positive figure, and they are characterised by having their outlines in 

 relief, while the rays of the positive figure are sunk below the plane 

 of the surface. When the electrification is strong, the ring enclosing 

 the rays stands up as a frill in considerable relief. 



"Effect of the Form of the Terminal. — The character, both of the 

 positive and negative figures, is greatly affected by the form of the 

 discharging electrodes. When the upper terminal is a metallic disc 

 of 25 mm. diameter, hanging in a plane parallel to, and 2 mm. 

 above, the dielectric surface, and the other terminal is a 50-mm. 

 metallic disc supporting the basin, complicated, but nearly symmetri- 

 cal, figures of great beauty are obtained. A metal point opposed to a 

 metal point, or a metal point above and a small metal ball below, 

 give smaller figures of more elementary forms, having the general 

 characteristics of the larger figures. 



Balls and points as terminals tend to produce circular figures in 

 which the rays converge to, or diverge from, one centre. This rule 

 applies with fewer exceptions to the + figure. The negative figure, 

 even when produced by a discharge from a brass ball, is frequently a 

 combination of sectors, whose centres are not far apart, and are concen- 

 tric with the centre of the group. The effect of this is to produce a 

 figure of nearly circular outline broken by more or less regular inden- 

 tations. 



If the ordinary vibrating contact-breaker be employed instead of 

 the trigger-break used in the foregoing experiments, the rest of the 

 arrangement remaining as described, larger and more complicated 

 effects are obtained. When the ball above the liquid is positive, the 

 resin and oil being at a temperature of 20° C, on breaking the 

 primary circuit the first effect of the make-and -break is the produc- 

 tion of the characteristic star with arborescent rays ; the repetitions 

 of the impact which instantly follow indent the lines of the 

 figure more and more deeply, and result in the effacement of the 

 more regular figure, and the development of a large and turbulent 

 movement of the liquid, tending to its division into two masses: a 

 central mound with a flat or concave top and a concentric ring. At 

 first the two masses are joined by radial ridges, but these gradually 

 thin, and (if the discharges at the spark-gap are continued) eventually 

 break down and leave the central mound and the embracing ring com- 

 pletely separated. During the progress of the action the ring por- 

 tion is driven outwards, and when the limit of outward movement is 

 reached, there is a subsidence of the more violent agitation, the outer 

 ring becomes somewhat smoother and flows inwards ; this is followed 

 by a recurrence of the repellent action, and a repetition of the last 



