ACOUSTICS AND GRAVITATION. 9 



Similar experiments were made with small induction coils. The fringes 

 here were always in motion, with relatively very large pulses recurring at 

 intervals. 



9. Specific inductive capacity. In equation 7, if the space d' is filled with 

 air, K' = i. On the other hand, if a plate of some insulator like glass is inserted 

 of thickness d' K 



(8) d' = d 



where d' & is the thickness of the air-layer. Moreover, if K g is the specific 

 inductive capacity of the insulator, 



If, therefore, in the absence of the insulator, y is the downward displacement 

 of the upper plate which gives the same fringe displacement , and hence 

 the same AV as the insertion of the insulator-plate, the two equations of the 

 form (6) (in the second of which y = Q, but K' as in equation (9)) are equal. 

 Hence 



-^Qd"y _4*Qd'd"f i ___ i \ 



A(K"d'+d") A \K'(K"d'+d" ~ K"d'+d" ) 



or inserting (9) 



or 



(ix) K* = d 



To determine the specific inductive capacity of a given insulating plate, the 

 electrophorus is discharged at a convenient distance d' between plate and 

 hard-rubber face. The insulator (K g ) is then inserted (noting the fringe 

 displacement n) and withdrawn. The fringes must then return to zero, show- 

 ing that no charge has been imparted by the friction of the insulator. The 

 upper plate is now depressed (y) on the micrometer-screw until the same 

 fringe displacement n is obtained. Equation (n) is thus applicable. The 

 operation should be quite rapid. 



The following are examples of this method, among many experiments made, 

 most of which proved quite disappointing. Thus, in the case of different plates 

 of glass, 



d' d e y x K 



