QUANTITY OF ELECTRIFICATION 27 



and charged by a powerful machine till " large sparks and brushes 

 were darting off from every part of its outer surface. 11 Faraday 

 " went into the cube and lived in it, and using lighted candles, 

 electrometers, and all other tests of electrical states, 11 he was unable 

 to detect any charge within the cube. 



Faraday devised a very simple and interesting method of 

 showing that charge resides only on the outer surface of a con- 

 ductor by an experiment known as the " butterfly-net experiment. 11 

 A conical muslin bag (Fig. 20A) which may be 10 or 12 inches 

 long is fitted on to a ring about 4 inches in diameter, placed on 

 the top of an insulating pillar with a base fixed to the table. The 

 muslin is preferably slightly starched. After it is starched two 



FIG. -20 A. 



insulating -silk threads air attached to the apex, one inside, one 

 out, so that the bag may be easily turned inside out merely by 

 pulling one of the threads. A charge is then communicated to 

 the bag, and it can be detected on a proof plane which has been 

 applied to the outer surface while no charge is thus detected on 

 the inner surface. The bag is then polled inside out by one of 

 the silk threads, and the outside, which was previously the inside, 

 is found to possess the whole charge. 



When (in in xulntcd charged conductor is brought within a hollow 

 conductor It induces an equal and opposite charge on the inside of the 

 hollow conductor, while an equal like charge goes to the outside. 



Let Fig. 21 represent a deep can on the table of an electro- 

 scope and let a positively charged insulated conductor be inserted 

 within the can. Outside the can and in the leaves there is a 

 positive charge, as may be shown by the collapse of the gold leaves 

 on bringing near the can a negatively electrified body, say an 

 ebonite rod. In>ide the can there is a negative charge, for if we 

 touch the can to earth to discharge the outside positive electrifi- 

 cation and then withdraw the charged body from within, the 



