SPECIFIC INDUCTIVE CAPACITY 



129 



Specific inductive capacity of water, alcohol, and 

 other electrolytes. The ordinary condenser method, with com- 

 paratively slow charge and discharge, is quite inapplicable to such 

 substances as water, in which the conduction is very appreciable. 

 The difficulty introduced by conduction was first overcome by Cohn 

 and Arons,* who used a modification of a method previously applied 

 to insulating liquids by Silow. If a quadrant electrometer has 

 one pair of quadrants to earth and the other pair is connected to 

 the needle and to a source giving potential V n , then the deflec- 

 tion (see p. 91) is approximately 



a c v ,. 2 

 : -\ T 



where C is the capacity of the needle per radian and A is the 

 torsion couple per radian that is, it is independent of the sign 

 of V n . If V n alternates rapidly is proportional to the mean 

 square of V n . 



If the medium between the needle and the plates is not air, but 

 a liquid with dielectric constant K, then 



KC V n * 



T T- 



Cohn and Arons used two electrometers and the alternating 

 potential supplied by one terminal of a Helmholtz induction coil. 

 When both contained air the deflections were, say, S t and $ 2 . 

 The second was then filled with the liquid to be experimented on, 

 the first still containing air. The deflections were now, say, 

 <V and J 8 '- 



In the first case we have 



* Wied. Ann., xxxiii. (1888), p. 1:5. 



