Electricity. 335 



power ) times that of copper. In the following list each substance con- 

 ducts better than any one of higher number : 



Good Conductors. Non-conductors or Insulators. 



1 Silver. 11 Oils. 



2 Copper. 12 Porcelain. 



3 Other metals. 13 Wool. 



4 Charcoal. 14 Silk. 



5 Water. 15 Resin. 



16 Gutta-percha. 

 Partial Conductors. 17 Shellac. 



6 The body. 18 Ebonite. 



7 Cotton. 19 Paraffin. 



8 Dry wood. 20 Glass. 



9 Marble. 21 Dry air. 

 10 Paper. 22 Vacuum. 



Ice, crystals, and solidified electrolytes are insulators. Naptha, tur- 

 pentine and some oils are insulators not the most perfect. 



The electric-glow occurs when a conductor terminates in a point, pro- 

 jecting into air and not near another conductor. The tension compels 

 a discharge into the air, into which the electricity, spreading in propor- 

 tion to the square of its distance, soon reaches a point, or rather a sur- 

 face, of air whose resistance prevents further dispersion, except as the 

 air itself is driven away, creating an electrical wind. Such passage of 

 electrified particles is called electrical convection. 



The electric brush consists of ramifying discharges into the air from 

 a blunt point or a small ball. The tension in this case diminishes less 

 rapidly with distance from the conductor, than in the other case. 



Electric tension is the stress or strain in a conductor of accumulated 

 electricity against an insulator or dielectric which bars its exit. 

 " Thomson has found that air, at the ordinary pressure and tempera- 

 ture, can support an electric tension of 9, 600 grains weight per square 

 foot, before a spark passes." ( Maxwell.) 



Induction is the creation of tension and polarity in a medium by 

 means of an electromotive force in a neighboring body. They both 

 cease upon the removal of the body in which the electromotive force is 

 displayed. Notwithstanding the fact mentioned above, that the space 

 in a vessel completely exhausted of air, is about the most effectual non- 

 conductor known, the power of induction across such space is not less- 

 ened a particle. This emphatically points to a " medium " remaining 

 in space after all ponderable and visible bodies are removed from it. 

 This medium is called ether by physicists, and its extension throughout 

 space is attested by the fact that electrical induction takes place across 

 the empty space between the sun and earth, a distance of 94, 000, 000 



