408 Professor A. W. Biicker on Electrical Stress. [Feb. 15, 



zontal, the field is dark until the cylinders are electrified, when light 

 is restored in the space between them. 



If parallel plates with carefully rounded edges, and about 2 milli- 

 metres apart, are used, the colours of Newton's rings appear in turn, 

 the red of the third order being sometimes reached. If one plate is 

 convex towards the other, the colours of the higher orders appear in 

 the middle, and travel outwards as the stress is increased. The 

 experiments may be varied by using two concentric cylinders, or two 

 sheets of metal bent twice at right angles to represent a section 

 through a Leyden jar. In the first case a black cross is formed ; and 

 in the second, black brushes unite the lower angles of the images of 

 the edges of the plates. By the interposition of a piece of selenite, 

 which shows the blue of the second order, two of the quadrants con- 

 tained between the arms of the cross become green, and the others 

 red. In like manner the horizontal and vertical spaces between the 

 inner and outer coatings of the "jar" become differently coloured. 



There are several phenomena connected with the stress in insu- 

 lators which present considerable difficulties. Thus it is found 

 impossible to restore the light between crossed Nicols by subjecting 

 a solid placed between them to electrical stress in a uniform field. 

 That the non-uniformity of the field has nothing to do with the 

 phenomenon in liquids, though at first disputed, is now generally 

 admitted. It may be readily proved by means of a Franklin's pane, 

 of which half is pierced with windows. The glow is much weakened 

 by thus replacing a uniform by a non-uniform field. 



Again, though most dielectrics w^hen placed in an electric field 

 expand, the fatty oils contract. Prof. J. J. Thomson has recently 

 pointed out that this indicates that another set of strains are super- 

 posed upon those assumed in the ordinary explanations of these 

 phenomena, and by which they may be neutralised or overcome. 



In experiments with carbon bisulphide it is necessary to take 

 every precaution against fire. For this purpose the cell which contains 

 the liquid should be immersed in a larger cell, so that if — as sometimes 

 happens — the passage of a spark cracks the glass the liquid may flow 

 into a confined space. This should stand in a tray with turned-up 

 edges, and an extinguisher of tin plate should be at hand to place 

 over the whole aj)paratus. No Leyden jars should be included in 

 the electrical circuit. The difficulties which formerly arose in the 

 exhibition of experiments in statical electricity owing to the presence 

 of moisture in the air of a lecture-room are now immensely reduced 

 by the Wimshurst machine, which works with unfailing certainty 

 under adverse conditions. A new and very beautiful machine was 

 kindly lent by Mr. Wimshurst for the purposes of the lecture. 



[A. W. R.] 



