ELECTRICITY. 



381 



electricity will escape, and the positive electricity alone remains, which is 

 coiiibiued with the negative electricity of the cake of resin, so long as the 

 cover is in contact with it. If we now remove the cover by its insulating 

 handle, the positive electricity, which was before held at the lower part of 

 the cover by the inductive action of the resin, will become free, and may be 

 imparted to any insulated conductor adapted to receive it. The same pro- 

 cess may be repeated indefinitely, as the resinous cake loses none of its elec- 

 tricity, but simply acts by induction, and thus an insulated conductor may bo 

 charged to any extent. 



751. An Electroscope is an instrument em- 

 ployed to indicate the presence of free elec- 



What is ai 

 Electroscope ? 



tricity. 



What is the 

 construction of 

 an electroscope 1 



Fig. 319. 



It usnally consists of two light conducting 

 bodies freely suspended, which in their natural 

 state hang vertically and in contact. When 

 electricity is imparted to them, they repel each other, and 

 the amount of their divergence is proportioned to the 

 quantity of electricity diffused on them. 



The simplest form of the electroscope, called the "pith-ball electroscope," 

 consists of two pith-balls suspended by silk threads. "When an excited body 

 is presented, the balls will be first attracted, but immediately acquu-ing the 

 same degree of electricity as the exciting body, they repel each other. An- 

 other form of the pith-ball electroscope, represented at B, Fig. 319, consists of 

 two pith-balls suspended by conducting threads within a glass jar, and con- 

 nected with the brass cap, vi. On touching the brass cap with an electrified 



body, the two balls being similarly electri- 

 fied, wiU repel each other. C, Fig. 319, 

 represents a more delicate electroscope; 

 two slips of gold leaf g g', being substituted 

 for the pith-baUs. If an excited substance, 

 e, be brought near the cap of brass, the 

 leaves will instantly diverge. The best 

 electrometers are carefully insulated so that 

 the electricity communicated to the balls or 

 leaves may not be too soon dissipated. 



Electroscopes merely indicate 

 the presence of an electrically excited body : they do not 

 measure the quantity, either relatively or absolutely, of 

 thQ electricity in action. 



752. An Electrometer is an instrument for 

 measuring the quantity of electricity. 

 The most simple form of the electrometer is represented at A, Fig. 319. It 



What is an 

 Electrometer ? 



