PHYSICAL FORCES 121 



Non-conductors are also called insulators, because they 

 serve to insulate electrified bodies, as the silk thread 

 insulates the pith balls and so keeps their electricity 

 from passing away. Air, especially dry air, is a bad 

 conductor, and this is why electricity, when the tension 

 becomes too strong, flashes through it as a spark, instead 

 of being quietly conducted by it from one electrified body 

 in one state to another body in the opposite electrical 

 condition. But air is not an absolute non-conductor, and 

 hence it is that by degrees the two similarly electrified 

 pith balls slowly part with their special electricity and 

 therewith repulsion ceasing fall together. 



Bodies in which electricity can be easily excited (e.g., 

 glass and sealing-wax) are in general the worst conductors, 

 but it seems that it can be excited in all bodies by friction 

 and, as we shall see later, by other means also ; only in 

 very many cases it runs away by conduction as quickly 

 as it is generated. But whenever so good a conductor 

 as a metal rod is fitted with a glass handle and so 

 insulated, it can be excited by rubbing and will retain its 

 electricity. 



The rapidity with which the electrical energy travels 

 is enormous, the velocity of electrical disturbance (or 

 energy) being the same as that of light.* 



The facts of conduction and insulation enable us to 

 accumulate electrical energy. The most familiar form 

 of its accumulation is in what is known as a Leyden Jar. 

 This is a glass jar, or wide bottle, all but the upper part 

 of -which is coated both inside and outside with tinfoil. 

 The glass keeps each of the two layers of tinfoil insulated 

 from the other, and the inner layer is entirely insulated. 

 The neck of the jar is then closed with a cork, through 



* See ante, p. 103. 



