314 ELECTRICAL INDUCTION. [SKCT. xxvin. 



each other. The attraction between electrified and unelec- 

 trified substances is, therefore, merely a consequence of their 

 altered state, resulting 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. Consequently, 

 the attraction exerted by an electrified substance upon an- 

 other substance previously neutral will be much more ener- 

 getic if the latter be a conductor than if it be a non-conductor. 

 The law of electrical attraction and repulsion has been de- 

 termined 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 according to the distance of the globe. A comparison 

 of the number of oscillations performed in a given time at 

 different 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 torsion of a thread, which con- 

 sequently measures their intensity ; and Mr. Snow Harris has 

 recently constructed an instrument with which he has mea- 

 sured 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. 

 However, the law of the repulsive force is liable to great 

 disturbance from inductive action, which Mr. Snow Harris 

 has found to exist not only between a charged and neutral 

 body, but also between bodies similarly charged ; and that, in 

 the latter case, the inductive process may be indefinitely mo- 

 dified by the various circumstances of the quantity and inten- 

 sity of the electricity, and the distance between the charged 



