38 Prof. G-. Quincke On Electrical Expansion, 



fully electrified, like a Franklin's plate, there is no double 

 refraction, as was known to me from former experiments, and 

 as also Gordon* and Mackenzie f have found. 



The glass is exposed to nearly equal electric force at all 

 points, and uniformly expanded; it is therefore no more 

 doubly refracting than equally heated glass. 



But if one of the tinfoil coatings be replaced by mercury 

 in a glass tube of 30 millims. exterior diameter and 14 millims. 

 inner diameter, the end of which has been ground and care- 

 fully cemented to the glass plate so that only the glass under 

 the mercury is electrically expanded, the glass is then sub- 

 jected to unequal electric action, and becomes doubly re- 

 fracting. 



37. If a fluid has heat communicated to particular portions 

 by means of a hot metal plunged in it more quickly than can 

 be conveyed away by conduction and convection, it becomes 

 optically doubly refractive, in the same way as unequally 

 heated glass. 



In the same way, a fluid between two metallic electrodes 

 becomes doubly refractive w^hen these are maintained at un- 

 equal electric potential. The unequal electric expansion is 

 dependent upon the velocity with which the electricity dif- 

 fuses itself in the fluid and produces the electric expansion. 

 The expansion is nearly proportional to the square of the 

 electric force active at a given point of the insulator. The 

 expansion must therefore be the greatest along the shortest 

 line of electric force between the electrodes. 



Substances whose refractive index increases with increase 

 of temperature, such as glass, and those whose refractive index 

 decrease, such as sulphide of carbon, will behave differently 

 if they are both expanded by electricity and if thermic and 

 electric expansion alter optical properties in the same direc- 

 tion. 



In fact, glass and sulphide of carbon show opposed electric 

 double refraction according to the observations of Kerr — a 

 result which my experiments have confirmed. 



Further, when substances (such as sulphide of carbon and 

 colza oil) are oppositely affected by electric force, both have 

 their refractive index diminished by thermic expansion ; then, 

 again, they must show opposed electric double refraction. 



This conclusion also is confirmed by experiment. 



38. If a piece of tinfoil be heated by an electric current 

 while between two homogeneous glass cubes, the glass becomes 

 doubly refractive, as if it expanded at right angles to the tin- 



* Phil. Mag. (5) ii. p. 203, 1876. t Wiedem. Ann, ii. p. 356, 1877. 



