6 Electrostatics Physical Principles [OH. i 



have the power of destroying the electrification of the brass rod when placed 

 in contact with it, while silk and glass and air do not possess this property. 

 It is for this reason that in handling the electrified brass rod, the substance 

 in direct contact with the brass has been supposed to be glass and not the 

 hand. 



In this way we arrive at the idea of dividing all substances into two 

 classes according as they do or do not remove the electrification when touch- 

 ing the electrified body. The class which remove the electrification are 

 called conductors, for as we shall see later, they conduct the electrification 

 away from the electrified body rather than destroy it altogether ; the class 

 which allow the electrified body to retain its electrification are called non- 

 conductors or insulators. The classification of bodies into conductors and 

 insulators appears to have been first discovered by Stephen Gray (1696- 

 1736). 



At the same time it must be explained that the difference between 

 insulators and conductors is one of degree only. If our electrified brass rod 

 were left standing for a week in contact only with the air surrounding it and 

 the glass of its handle, we should find it hard to detect traces of electrifica- 

 tion after this time the electrification would have been conducted away by 

 the air and the glass. So also if we had been able to immerse the rod in a 

 flame for a billionth of a second only, we might have found that it retained 

 considerable traces of electrification. It is therefore more logical to speak of 

 good conductors and bad conductors than to speak of conductors and insula- 

 tors. Nevertheless the difference between a good and a bad conductor is so 

 enormous, that for our present purpose we need hardly take into account the 

 feeble conducting power of a bad conductor, and may without serious incon- 

 sistency, speak of a bad conductor as an insulator. There is, of course, nothing 

 to prevent us imagining an ideal substance which has no conducting power 

 at all. It will often simplify the argument to imagine such a substance, 

 although we cannot realise it in nature. 



It may be mentioned here that of all substances the metals are by very 

 much the best conductors. Next come solutions of salts and acids, and lastly 

 as very bad conductors (and therefore as good insulators) come oils, waxes, 

 silk, glass and such substances as sealing wax, shellac, indiarubber. Gases 

 under ordinary conditions are good insulators. Indeed it is worth noticing 

 that if this had not been so, we should probably never have become acquainted 

 with electric phenomena at all, for all electricity would be carried away by 

 conduction through the air as soon as it was generated. Flames, however, 

 conduct well, and, for reasons which will be explained later, all gases become 

 good conductors when in the presence of radium or of so-called radio-active 

 substances. Distilled water is an almost perfect insulator, but any other 

 sample of water will contain impurities which generally cause it to conduct 



