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



The dielectric was either contained in glass basins or spread as a 

 coating of 0*5 to 1 mm. thick upon glass or mica plates, and in a few 

 cases on copper plates. Also plates consisting wholly of resin were 

 in some instances nsed. 



The effect of a spark passing at the spark-gap, when one of the 

 stage terminals is suspended over, and at a certain distance from the 

 viscid resin and oil mixture contained in a basin, the other being in 

 contact with a metallic disc under it, is to produce an evanescent 

 figure on the surface. 



The character of the figure depends on : — 



1. Whether the terminal over the dielectric surface is positive or 



negative. 



2. The form and size of the + and — terminals. 



3. The distance of the upper terminal from the snrface of the 



dielectric. 



4. The potential and character of the spark at the spark-gap. 



Typical Effects. — The most regular and characteristic stress figures 

 are obtained when the spark-gap is adjusted so as to prevent the 

 passage of a spark or visible brush through or over the dielectric, 

 but allow a non-luminous discharge to take place of only slightly less 

 strength than would be necessary to produce a brush discharge 

 visible in the dark. A typical effect is obtained when the spark-gap 

 is 25 mm., and the positive branch from, it terminates in a brass ball of 

 8 mm. diameter hanging centrally over, and 4 mm. from the surface 

 of, the dielectric (80 per cent, resin and 20 per cent, resin oil at 

 20° C.) contained in a glass basin 150 mm. diameter and 15 mm. 

 deep, the negative wire being brought to a disc of metal 100 mm. 

 diameter under the basin, or to a disc of tinfoil attached to the 

 underside. On breaking the primary circuit by means of a mercury 

 break with a trigger action (the spark-gap having been momentarily 

 short-circuited while the primary circuit was closed), and the con- 

 sequent passing of a single spark at the spark-gap — no visible dis- 

 charge occurring between the ball and the dielectric — there suddenly 

 breaks out on the surface of the viscous liquid a star-shaped figure 

 formed of deeply furrowed, closely clustered, outward-branching rays, 

 extending from a circular frill near the centre to the margin of the 

 liquid. The figure gradually dies down, and on the surface becoming- 

 smooth it can, with slight variations, be reproduced again and again 

 by repeated breaks of the primary circuit. 



If the commutator is reversed (the spark-gap being momentarily 

 closed while the primary circuit is re-made), then on breaking the 

 primary circuit as before, a figure characteristic of the negative con- 

 vective discharge is produced. This figure is much smaller and 

 weaker than the positive one ; most frequently it consists of a 



