122 ELEMENTS OF SCIENCE 



which passes a piece of metal (wire or a chain) which 

 internally makes contact with the inner layer of foil, 

 while externally it terminates in a rounded knob pro- 

 jecting freely from above the cork of the jar. 



Then electricity is produced by that familiar instrument 

 called an electrical machine, which consists of a disc, or a 

 cylinder, of glass, capable of being rapidly turned between, 

 and so becoming rubbed by, two cushions ; such action 

 (as in the experiment with the silk and the glass rod) 

 developing electricity on the glass. This is collected, as 

 it is generated, by certain metal parts of the machine, 

 which are insulated on glass legs and which end in a 

 projecting knob, whence in very powerful machines 

 sparks may sometimes be obtained half a yard long and 

 strong enough to knock a man down. 



Electricity having been thus generated, it is carried by 

 a conductor to the knob on the Leyden jar, whence it 

 passes, through the wire, or chain, to the inner coating 

 of foil * which becomes charged with (positive) electricity 

 when the outer coating of the jar also becomes, by in- 

 duction, simultaneously charged with electricity of the 

 opposite kind in this case negative. Then if a piece of 

 metal (with a glass handle for safety from shock) be 

 made to touch any part of the outer layer of foil with 

 one end, while its other end is applied to the knob of 

 the jar, an instantaneous discharge of electricity takes 

 place, and the two electricities neutralise each other and 

 disappear. Before contact is effected, a brilliant spark 

 with a more or less loud report, will pass between the 



* The electricity is thus by no means stored in the bottle, as 

 would be the case with a liquid. That, when charged, the 

 particles which compose the glass assume a peculiar arrange- 

 ment is a favourite hypothesis. 



