144 STATIC ELECTRICITY 



Quincke also observed a decrease in the torsipnal rigidity ot 

 glass, mica, and india-rubber when electrically strained. 



Electric double refraction: The Kerr effect. Km 

 discovered* that a liquid dielectric is in general doubly refracting 

 when it is the seat of electric strain. It behaves as a uniaxial 

 crystal with the optic axis along the lines of force. There is there- 

 fore a difference in the velocity of rays which traverse the liquid 

 perpendicular to the lines of force, according as their plane of 

 polarisation is parallel to or perpendicular to the line- of force. 

 The effect may be shown by the arrangement of which I-'ig. D~> 

 is a plan. 



A ray of light is sent through a polariser P set so that its plain of 

 polarisation is at 45 to the vertical. It then passes into ft trough 



T with glass ends containing the liquid, winch may be carbon 

 bisulphide very pure and dry. In the trough arc t\\o bra ex Under- 

 3 in. or 4 in. long, with their axes parallel and in the -mm- hoi i/ontal 

 plane. These are connected by rods rising out of the trough to the 

 terminals of a Wimshurst machine. After emerging from tin- 

 trough the ray passes through the analyser A, set at 90 to the 

 polariser, so that when the liquid is not electrically strained there 

 is complete extinction. The ray then passes on to the eye of an 

 observer E, or by using lenses we may project an image of the end - 

 of the conductors in the trough on to a screen. On working the 

 Wimshurst the field betxveen the two conductors traiiMiiit- the light 

 to some extent, and the more the stronger the field. The vibration 

 of the incident beam may be resolved into equal components 

 vibrating respectively vertically and horizontally, and therefore 

 respectively along and perpendicular to the lines of force. Ti 

 travel at different speeds through the strained medium so that 

 when they emerge from the electric field the phases differ, and 

 instead of uniting to form a plane polarised beam, with pi 

 parallel to that of the original, they unite to form an elliptically 

 polarised beam which cannot be entirely extinguished by the 

 analyser. By inserting a Jamin compensator C, Kcrr determined 



* Phil. Mag. [4], 1875, vol. 1. pp. 337 and 446 ; [5], 1879, vol. viii. pp. 85 and 229. 



