INTRODUCTORY. 



The distinction between good and bad conductors does not really 

 correspond to any essential difference of properties. Electricity moves 

 upon all bodies with greater or less freedom. No bodies are known 

 which are absolute -insulators that is to say, on which electrical pro- 

 perties can be retained for an indefinite time without alteration. 



In like manner, notwithstanding the rapidity with which elec- 

 tricity is transmitted on the best conductors, there are none on 

 which it is propagated instantaneously. Very material differences 

 are found among them in this respect, and we can determine the 

 special resistance which each one offers to the motion of electricity. 



3. Two ELECTRICITIES. When any two bodies are rubbed 

 together, a piece of glass and a piece of resin for instance, both 

 become electrified, but with different characters ; each of them 

 repels an insulated light body which has been in contact with it, 

 and which has shared its electricity ; but the resin attracts the body 

 which has been touched by the glass, and the glass the body which 

 has been touched by the resin. 



The condition of the glass differs, then, from that of the resin, 

 which is expressed by saying that the electricity of the glass is of 

 a different kind to that of the resin. Experiment shows, moreover, 

 that any electrified body behaves either like the glass, or like the 

 resin of the preceding experiment. It attracts, for instance, the 

 body electrified by glass, and repels that which has been electrified 

 by resin, or conversely. There are thus two kinds of electricity, and 

 only two. This fundamental property may be formulated by saying 

 that two bodies charged with the same electricity repel each other, and 

 two bodies charged with opposite electricities attract each other. 



4. ELECTRICAL ACTIONS. ELECTRICAL MASSES. The action 

 exerted between two electrical bodies whose dimensions are small 

 in comparison with their distance apart, is in the direction of the 

 straight line joining them. 



Coulomb found by direct experiment that this force is inversely 

 as the square of the distance. It is also a function of the electrical 

 condition of adjacent bodies, or of their electrification. 



If, between two identical bodies of very small dimensions, and 

 placed at unit distance, the electrical action is equal to unit force, 

 the quantity of electricity, or the electrical mass of each of them, is 

 equal to unity. 



If, while the condition of one of the bodies remains unchanged, 

 the distance being also unchanged, the action between them becomes 

 2, 3 ... times as great, the electrical mass of the other is said to 

 have become 2, 3 ... times as great 



