1889.] on Electrical Stress. 407 



of the cylinder, becomes electrified by contact, and is repelled down 

 its axis. In view of this explanation, and the movements assumed 

 can be clearly seen in the j^henyl thiocarbamide, the performance of 

 the exiDeriment on a small scale is not without interest. The possi- 

 bility of the formation of such violent up-and-down currents in so 

 small a space must depend upon a very nice adjustment between the 

 properties of the liquid and the forces in play. It is also obvious that 

 such movements of the liquid must be a disturbing element in any 

 attempt to make the lines of electric force visible. 



Again, if a solid powder be suspended in a liquid into which 

 electrified solids are introduced, it tends to accumulate round one of 

 the poles. This subject has been investigated by W. Holtz. Some- 

 times the powder appears to move in a direction opposed to that in 

 which the liquid is streaming. Sometimes two powders will travel 

 towards different poles. 



If powdered antimony sulphide be placed in ether, it settles at the 

 bottom of the liquid, and if either two wires insulated with glass up 

 to their points, or two vertical plates be used as electrodes, and 

 slightly electrified, the solid particles arrange themselves along 

 the lines of force. If the electrification be increased, they cluster 

 round the positive pole. On suddenly reversing the electrification 

 by means of a commutator, they stream along lines of force to the 

 pole from which they were previously- repelled. Other methods of 

 obtaining the lines of force have been devised. They can, for instance, 

 be shown by crystals of sulphate of quinine immersed in turpentine. 



The tendency of the lines of force to separate one from the other 

 was illustrated by Quincke's experiment. A bubble of air is formed 

 in bisulphide of carbon between two horizontal plates. It is in connec- 

 tion with a small manometer, and when the plates are oppositely 

 excited, the electrical pressure acting at right angles to the lines of 

 force, being greater in the liquid than in air, compels the bubble to 

 contract and depresses the manometer. 



Kerr's experiments depend upon the fact that, since the electrical 

 stress is a tension along the lines of force, and a pressure at right 

 angles to them, a substance in which such a stress is produced assumes 

 a semicrystalline condition in the sense that its properties along, and 

 perpendicular to, the lines of force are different. Light is therefore 

 transmitted with different velocities according as the direction of 

 vibration coincides with, or is perpendicular to, these lines ; and the 

 familiar j)henomena of the passage of polarised light through crystals 

 may be imitated by an electrically stressed liquid. 



The bisulphide of carbon used must be dry, and, to make the 

 phenomena clearly visible, it is necessary that the light should travel 

 through a considerable thickness. Thus, to represent the stress 

 between two spheres, elongated parallel cylinders should be used, the 

 axes of which are parallel to the course of the rays of light. These 

 appear on the screen as two dark circles. Between crossed Nicols, 

 the planes of polarisation of which are inclined at 45° to the hori- 



