INDUCTION. 53 



If that divergence has increased, then the power of this 

 insulator to transmit electric influence that is, its spe- 

 cific inductive capacity is greater than that of air; 

 otherwise, it is equal to, or less than that of air. 



From this we see that specific inductive capacity varies 

 inversely as insulation. Hence this property is almost 

 infinite in the best conductors ; while in the best insu- 

 lators it is the reverse. 



By methods similar to the above, with the aid of 

 improved instruments, to be described hereafter, the 

 specific inductive capacities of a number of substances, 

 including the principal insulators, have been carefully 

 estimated by Boltzmann, Gordon, and others: and from 

 the results obtained by them the table on the next page 

 has been prepared, in which the general averages are 

 given. 



The results obtained by different observers differ so 

 widely that they can only be regarded as approximate, 

 and will undoubtedly require future correction, when 

 improved methods shall give greater accuracy. 



The table shows the electric resistance of glass to be 

 much less than that of ebonite ; the inverse ratio being 

 5.87 to 2 89 : and this is doubtless true of glass, in the 

 average. But, if the best insulating glass were com- 

 pared with the best insulating ebonite, the ratio might 

 require to be reversed. Ebonite, when subjected to a 

 powerful electric strain, seems to yield gradually, and 

 allow the electricity to creep through it ; and, by con- 

 tinued strain, its electric resistance soon becomes 

 permanently impaired : while the best insulating glass 

 rigidly resists, and suffers fracture before yielding. 



But, according to Gordon, the electric resistance of 

 glass also becomes somewhat impaired by long use ; or, 



