490 ON PHYSICAL LINES OF FORCE. 



According to our theory, the particles which form the partitions between 

 the cells constitute the matter of electricity. The motion of these particles 

 constitutes an electric current; the tangential force with which the particles 

 are pressed by the matter of the cells is electromotive force, and the pressure 

 of the particles on each other corresponds to the tension or potential of the 

 electricity. 



If we can now explain the condition of a body with respect to the 

 surrounding medium when it is said to be "charged" with electricity, and 

 account for the forces acting between electrified bodies, we shall have established 

 a connexion between all the principal phenomena of electrical science. 



We know by experiment that electric tension is the same thing, whether 

 observed in statical or in current electricity; so that an electromotive force 

 produced by magnetism may be made to charge a Leyden jar, as is done by 

 the coil machine. 



When a difference of tension exists in different parts of any body, the 

 electricity passes, or tends to pass, from places of greater to places of smaller 

 tension. If the body is a conductor, an actual passage of electricity takes 

 place; and if the difference of tensions is kept up, the current continues to 

 flow with a velocity proportional inversely to the resistance, or directly to the 

 conductivity of the body. 



The electric resistance has a very wide range of values, that of the metals 

 being the smallest, and that of glass being so great that a charge of electricity 

 has been preserved* in a glass vessel for years without penetrating the thick- 

 ness of the glass. 



Bodies which do not permit a current of electricity to flow through them 

 are called insulators. But though electricity does not flow through them, 

 the electrical effects are propagated through them, and the amount of these 

 effects differs according to the nature of the body; so that equally good insu- 

 lators may act differently as dielectrics f. 



Here then we have two independent qualities of bodies, one by which they 

 allow of the passage of electricity through them, and the other by which they 

 allow of electrical action being transmitted through them without any electri- 

 city being allowed to pass. A conducting body may be compared to a porous 

 membrane which opposes more or less resistance to the passage of a fluid, 



* By Professor W. Thomson. t Faraday, Experimental Ittxearcftes, Series xi. 



