8 



PHYSIOLOGICAL PHYSICS. 



[Chap. I. 



glass shade, which rests on a metallic support. It is 

 used in the following way : If a charged body be 

 brought near, by induction the neutral fluid of the rod 

 and leaves is decomposed, and the kind of electricity 

 opposite to that of the charged body is 

 driven into the leaves, which diverge. 

 This shows that the body brought near 

 is charged. In this condition touch 

 the knob B with a finger, contact is 

 made with the ground and the free 

 electricity (i.e. that of the leaves) 

 escapes, and the leaves collapse. 



Now remove the finger from 

 the knob, and take away next 

 the charged body. The electricity 



Fig. 3. Gold-leaf. , i j i, AI * ^ 



Electroscope. kept bound by the presence of the 

 charged body, and of the opposite 

 kind to it, is now free, and diffuses itself over 

 the knob, rod, and leaves, which last again diverge. 

 To discover with what kind of electricity the inducing 

 body was charged, bring an electrified glass rod ( + ) 

 near the knob. If the leaves diverge still more, 

 because like electricities repel, it is positive electricity 

 that is in the electroscope, and then it must have 

 been electricity of the opposite kind ( ) with which 

 the body was charged. It is necessary to approach 

 the glass rod (or resin, if it be used) slowly, and to 

 accept the first movement made by the leaves as the 

 required indication. 



Electrical machines. The ELECTROPHORUS 

 (Fig. 4) is also an instrument acting by induction. 

 It consists of a cake of resin or ebonite, etc., B, fitted 

 into a metallic mould, and of a metal disc A smaller 

 than B, so as to rest upon it, and provided with an 

 insulating handle of glass. The resin, having been 

 warmed, is beaten with a cat-skin, which develops 

 electricity. The metal disc, called the cover, is then 



