84 



Observations on the Spark Discharge. 



may occur reversed in order in the same slide, i.e., the pattern may 

 point towards either pole.* 



(23 ) A few of the cracks which cross the spark tracks are seen on 

 the last figure and on the other figures. When the spark traverses 

 wider places cracks alone appear, and these are sometimes very regular 

 in appearance. It would appear that these are the result of electro- 

 static stresses in the glass exerted orthogonally to the lines of flow. 

 Thus, near the poles these cracks curve round the pole with a small 

 radius of curvature, the curves flatten as they are further removed 

 from the pole and in the centre of the field cross the line joining the 

 poles perpendicularly. Intersecting these orthogonally are the spark 

 tracks, which, in cases where the glasses are at a uniform but short 

 distance from one another throughout, or parallel, appear to pass in 

 elliptical lines from pole to pole. Fig. 2, PL 5, is a photo magnified 

 fifteen diameters of the field near the positive pole. It will be seen that 

 ifc recalls the usual figures of the distribution of equipotential lines 

 and lines of flow for such a disposition of the poles. It is remarkable 

 that near the negative pole the flow lines often curve outwards at 

 some little distance from the pole. If free sparks, taken from carbon 

 points or dusty terminals, be observed, the luminous particles of 

 burning carbon or dust will be seen to have the same outward curva- 

 ture at the — pole. Those at the + pole curve elliptically towards 

 the negative (fig. 4). 



Fig. 4. 



Path of incandescent particles at the poles. 



(24.) The homogeneousness of the glass is essential to all the fore- 

 going effects. A plate of Iceland spar was polished parallel to a 

 rhombohedral cleavage face. On this a thin cover glass was laid 

 down over platinum leads as before and sparks passed. Search for 

 tracks of the usual appearance on this was in vain ; different 

 phenomena presented themselves. The track was marked by innu- 

 merable little pits with raised, rounded edges, running on the 

 whole in cleavage directions, i.e., dividing up the surface into 

 rhombohedral pits. These pits look as if they had been melted into 



* The cloudy marking at either side of the track seen in the figure is due to 

 moisture which had penetrated between the glasses before the photograph was 

 taken. In the case of the other photographs the glasses were separated before 

 photographing, the figures being obtained from one of the glasses only. 



