22 ELEMENTARY LESSONS ON [CHAP. I. 



induction to explain why an electrified body should 

 attract things that have not been electrified at all. Let 

 a light ball be suspended by a silk thread (Fig. 1 1), and 

 a rubbed glass rod held near it. The positive electricity 

 of the glass will induce negative electricity on the near 

 side, and an equal amount of 

 positive electricity on the farther 

 side, of the ball. The nearer 

 half of the ball will therefore 

 be attracted, and the farther 

 half repelled ; but the attraction 

 will be stronger than the repul- 

 sion, because the attracted elec- 

 tricity is nearer than the repelled. Hence on the whole 

 the ball will be attracted. It can easily be observed 

 that if a ball of non-conducting substance, such as wax, 

 be employed, it is not attracted so much as a ball of 

 conducting material. This in itself proves that induction 

 really precedes attraction. 



21. Inductive capacity. We have assumed up to 

 this point that electricity could act at a distance, and 

 could produce these effects of induction without any 

 intervening means of communication. This, however, 

 is not the case, for Faraday discovered that the air in 

 between the electrified body and the conductor played a 

 very important part in the production of these actions. 

 Had some other substance, such as paraffin oil, or solid 

 sulphur, occupied the intervening space, the effect pro- 

 duced by the presence of the electrified body at the 

 same distance would have been greater. The power of 

 a body thus to allow the inductive influence of an 

 electrified body to act across it is called its inductive 

 capacity (see Article 49 and Lesson XXII.) 



22. The Electrophorus. We are now prepared 

 to explain the operation of a simple and ingenious 

 instrument, devised by Volta in 1775, for the purpose 

 of procuring, by the principle of induction, an unlimited 



