CHAPTER XI 



ALTERATIONS OBSERVED IN THE DIELEC- 

 TRIC WHEN IT IS SUBJECTED TO 

 ELECTRIC STRAIN 



Electric expansion in glass Maxwell's electric stresses do not explain 

 the effect Electric expansion of liquids Electric double refraction : 

 The Kerr effect. 



WHEV a dielectric is subjected to electric strain it shows in MHIH* 

 cases a change in volume and in other cases it becomes doubly 

 retracting. 



Electric expansion in glass. The dielectric in a glass 

 condenser generally expands when the condenser is charged, and 

 the dielectric becomes the seat of electric strain. This expansion 

 was known to Volta and has since been studied by Go\i, Duter, 

 Righi, and especially by Quincke.* It may be conveniently 

 observed and measured by using a common thermometer as a 

 Leyden jar. The bulb is immersed in melting ice and water, tin- 

 water serving as the outer coating, while the liquid contained in 

 the bulb serves as the inner coating. On charging the inside by 

 connecting it with a large Leyden jar the level is observed to fall. 

 Quincke found that if Ai> is the change in the internal volunx 

 the bulb, if e is the thickness of the glass and V is the potential 

 difference, then 



- is proportional to - 2 nearly. 



.With thicknesses between O'l mm. and 0'5 mm. and with a 

 potential difference sparking at 2 mm. between knobs '2 an. in 

 diameter, Quincke found that the change in volume lav between 1 

 in 10 5 and 1 in 10 7 . In general, when it rose above 1 in 10* 

 sparking occurred and the glass was perforated. 



The intensity and strain are proportional to V /e so that the 

 change in volume is proportional to the square of the electric strain 

 in the glass. The expansion occurs equally in all directions, for 



.simultaneous measurements of increase in length / and of volume v 



* Phil. May. [5], vol. 3, j>. 30, where references to earlier work will be found. 



