PHYSIOLOGICAL PHYSICS 



[Chap. I. 



accumulates in greater amount at the pointed ex- 

 tremities. Suppose one of these ends to be extremely 

 pointed, then the accumulation may be so great that 

 the electricity tends to pass off from the conductor ' 

 into the atmosphere. In other words, the TENSION at 

 that point will be considerable. 



Induction is the term applied to the influence 

 exerted by electrified upon uiielectrified bodies. 

 Suppose two conductors (Fig. 2) s and AB, both in- 

 sulated by being 

 mounted on glass 

 stands, to be 

 brought near to 

 one another, s 

 having received 

 a charge of posi- 

 tive electricity, 

 and AB being un- 

 charged. Suppose 

 AB to have attach- 

 ed to it three 



ELECTROSCOPES 



Fig. 2. Induction. 



a d b. They consist of a metallic stem fixed by 

 metallic contact to the conductor ; and from the upper 

 end of each stem there hangs a pith-ball suspended 

 by a linen thread. When AB is unelectrified, the 

 pith-balls are in contact with the stems from which 

 they hang. Should the conductor become charged, 

 the pith-balls will also become charged by contact, .and 

 since, then, the stem and the pith-ball are charged 

 with similar electricity, they will repel one another. 

 The divergence of the pith-ball, therefore, indicates that 

 the conductor has received a charge. The conductor, 

 being charged, is brought into the neighbourhood of 

 AB uncharged. At once the electroscopes indicate 

 the electrification of AB. Remove s, and the signs of 

 AB'S electrification disappear ; again bring s near, 



