228 ELEMENTARY LESSONS ON [CHAP. iv. 



As to the precise nature of the molecular or mechanical 

 operations in the dielectric when thus subjected to the 

 stress of electrostatic induction, nothing is known. One 

 pregnant experiment of Faraday is of great importance, 

 by showing that induction is, as he expressed it, " an 

 action of contiguous particles." In a glass trough (Fig. 



104), is placed 

 some oil of tur- 

 pentine, in which 

 are put some fibres 

 of dry silk cut into 

 _. small bits. Two 



Fig. 104. 



wires pass into 



the liquid, one of which is joined to earth, the other 

 being put into connection with the collector of an 

 electrical machine. The bits of silk come from all 

 parts of the liquid and form a chain of particles from 

 wire to wire. On touching them with a glass rod they 

 resist being pushed aside, though they at once disperse 

 if the supply of electricity is stopped. Faraday regarded 

 this as typical of the internal actions in every case of 

 induction across a dielectric, the particles of which he 

 supposed to be " polarised," that is, to be turned into 

 definite positions, each particle having a positive and a 

 negative end. The student will perceive an obvious 

 analogy, therefore, between the condition of the particles 

 of a dielectric across which electrostatic induction is 

 taking place, and the molecules of a piece of iron or 

 steel when subjected to magnetic induction. 



Siemens has shown that the glass of a Leyden jar is 

 sensibly warmed after being several times rapidly charged 

 and discharged. This obviously implies that molecular 

 movement accompanies the changes of dielectric stress. 



273. Electric Expansion. Fontana noticed that 

 the internal volume of a Leyden jar increased when it 

 was charged. Volta sought to explain this by suggesting 

 that the attraction between the two charged surfaces 



