of Electricity 47 



the string it was dynamic, but as soon as it 

 was stored in the Leyden jar it became static. 

 Current electricity is dynamic. A closed tele- 

 graphic circuit is charged dynamically, while 

 the prime conductor of a f rictional electric ma- 

 chine is charged statically. The distinction 

 is arbitrary and in a sense a misnomer. When 

 we rub a piece of hard rubber with a cat- 

 skin it is statically charged because the sub- 

 stances are what are called non-conductors, 

 and the charge cannot be conducted readily 

 away. All substances are divided into two 

 classes, to wit, conductors or non-electrics, and 

 non-conductors or electrics, more commonly 

 called dielectrics. These, however, are relative 

 terms, as no substance is either a perfect con- 

 ductor or a perfect non-conductor. 



The metals, beginning with silver as the 

 best, are conductors. Ebonite, paraffine, shel- 

 lac, etc., are insulators, or very poor conductors. 

 The best conductors offer some resistance to 

 the passage of the current and the best insu- 

 lators conduct to some extent. If we make a 

 comparison of electric conductors we find that 

 the metals that conduct heat best also conduct 

 electricity best. This, it seems to me, is a con- 

 firmation of the atomic theory of electricity 

 so far as it means anything. If a good con- 

 ductor, as silver, is subjected to intense cold 

 by putting it into liquid air, its conductivity 

 is greatly increased. It is well known that 



