220 ELEMENTARY LESSONS ON [CHAP. iv. 



pelled. The inconstancy of dry -piles as sources of 

 electrification led Hankel to substitute a battery of a 

 very large number of small DanielPs cells. 



265. Capillary Electrometers. The Capillary 

 Electrometer of Lippmann, as modified by Dewar, was 

 described in Art. 225. 



LESSON XXII. Specific Inductive Capacity ', etc. 



266. In Lesson VI. it was shown that the capacity 

 of a Leyden jar or other accumulator depended upon the 

 size of the conducting coatings or surfaces, the thinness 

 of the glass or other dielectric between them, and upon 

 the particular " inductive capacity " of the dielectric 

 used. We will now examine the subject in a more 

 rigorous way. In Art. 246 it was laid down that the 

 capacity of a conductor was measured by the quantity 

 of electricity required to raise its potential to unity ; or 

 if a quantity of electricity Q raise the potential from 

 V to V then its capacity is 



Now, a Leyden jar or other accumulator may be 

 regarded as a conductor, in which (owing to the parti- 

 cular device of bringing near together the two oppositely- 

 charged surfaces) the conducting surface can be made 

 to hold a very large quantity of electricity without its 

 potential (whether + or ) rising very high. The 

 capacity of an accumulator, like that of a simple con- 

 ductor, will be measured by the quantity of electricity 

 required to produce unit rise of potential. 



267. Theory of Spherical Air -Condenser. 

 Suppose a Leyden jar made of two concentric metal 

 spheres, one inside the other, the space between them 

 being filled by air. The inner one, A, will represent the 

 interior coating of tinfoil, and the outer sphere, B (Fig. 



