Scr. XXVllf. ELECTRICAL FORCES. 275 



induction. When a body in either electric state is pre- 

 sented to a neutral one, its tendency, in consequence of 

 the- law of induction, is to disturb the electrical condi- 

 tion of the neutral body. The electrified body induces 

 electricity contrary to its own in the adjacent part of 

 the neutral one, and therefore an electrical state similar 

 to its own in the remote part. Hence the neutrality of 

 the second body is destroyed by the action of the first, 

 and the adjacent parts of the two, having now opposite 

 electricities, will attract each other. The attraction be- 

 tween electrified and unelectrified substances is, there- 

 fore, merely a consequence of their altered state, re- 

 sulting directly from the law of induction, and not an 

 original law. The effects of induction depend upon the 

 facility with which the equilibrium of the neutral state 

 of a body can be overcome a facility which is propor- 

 tional to the conducting power of the body. Conse- 

 quently the attraction exerted by an electrified substance 

 upon another substance previously neutral, will be much 

 more energetic if the latter be a conductor than if it be 

 a non-conductor. 



The law of electrical attraction and repulsion has 

 been determined by suspending a needle of gum-lac 

 horizontally by a silk fibre, the needle carrying at one 

 end a piece of electrified gold-leaf. A globe in the same, 

 or in the opposite electrical state, when presented to 

 the gold leaf, will repel or attract it, and will therefore 

 cause the needle to vibrate more or less rapidly accord- 

 ing to the distance of the globe. A comparison of the 

 number of oscillations performed in a given lime at dif- 

 ferent distances, will determine the law of the variation 

 of the electrical intensity, in the same manner that the 

 force of gravitation is measured by the oscillations of 

 the pendulum. Coulomb invented an instrument which 

 balances the forces in question by the force of the tor- 

 sion of a thread, which consequently measures their 

 intensity ; and Mr. Snow Harris has recently construct- 

 ed an instrument with which he has measured the 

 intensity of the electrical force in terms of the weight 

 requisite to balance it. By these methods it has been 

 found that the intensity of the electrical attraction and 

 repulsion varies inversely as the squares of the distances. 



