202 EXPERIMENTAL GENERAL SCIENCE 



charged body) has flowed away to the earth as soon as touched. 

 There is left in the electroscope, therefore, only one kind of 

 electricity which causes the strips of foil to repel one another, 

 and until enough of the other kind of electricity can return to 

 the electroscope and balance the charge it contains, the strips 

 of foil must continue to be separated. This return of elec- 

 tricity is prevented by the glass case through which it can 

 not pass. 



166. Insulators. Substances which, like glass, prevent the 

 passage of electricity are called insulators. Glass is one of the 

 best of insulators. Others are silk, amber, sulphur, rubber, 

 and dry air. Metals and other good 

 conductors of heat are also good con- 

 ductors of electricity. Water is ordi- 

 narily a poor conductor, but when salts 

 are dissolved in it, it becomes a good 

 conductor. Substances that are ordi- 

 narily good insulators may become con- 

 ductors when wet. 

 FIG. 34. Leyden jar 167. The Leyden Jar. A glass jar 



and discharger. (Tower, with th lower ha j f CQated i ns id e and 

 Smith and Turton.) 



out with tinfoil is known as a Leyden 



jar. A metal rod held in place by an insulator extends down 

 inside of the jar and in contact with the lining. When the 

 knob at the top of the metal rod is charged with positive 

 electricity, it repels the positive electricity on the outside of 

 the jar, which is at once conducted from the jar to the earth 

 through the jar's support. After a time the jar becomes fully 

 charged and will then receive no more electricity. It now has 

 a charge of positive electricity on its inner surface and a like 

 charge of negative electricity on its outer surface. If the two 

 surfaces are then nearly connected by means of a wire, the jar 

 is then discharged with a bright spark and crackling noise. Such 

 jars and other similar structures are often known as condensers. 



