CHAP. i.l- ELECTRICITY AND MAGNETISM. 



29 



glass tube is rubbed the bullet acquires the property of 

 attracting light bodies. If a dry silk thread is used, 

 however, no electricity will flow down to the bullet. 



Gray even succeeded in transmitting a charge of 

 electricity through a hempen thread over 700 feet long, 

 suspended on silken loops. A little later Du Fay 

 succeeded in sending electricity to no less a distance 

 than 1256 feet through a moistened thread, thus proving 

 the conducting power of moisture. From that time the 

 classification of bodies into conductors and insulators 

 has been observed. 



Fig. 17. 



This distinction cannot, however, be entirely main- 

 tained, as a large class of substances occupy an inter- 

 mediate ground as partial conductors. For example, dry- 

 wood is a bad conductor and also a bad insulator ; it 

 is a good enough conductor to conduct away the high- 

 potential electricity obtained by friction ; but it i 

 bad conductor for the relatively low-potential electricity 

 of small voltaic batteries. Substances that are very bad 

 conductors are said to offer a great resistance to the 



