CHAP, i.] ELECTRICITY AND MAGNETISM. 



57 



Fig. 31 



the city where it was invented, is a convenient form of 

 accumulator. It usually consists (Fig. 31) of a glass jar 

 coated up to a certain height on the inside and outside 

 with tinfoil. A" brass knob 

 fixed on the end of a stout 

 brass wire passes downward 

 through a lid or top of dry 

 well - varnished wood, and 

 communicates by a loose bit 

 of brass chain with the inner 

 coating of foil. To charge 

 the jar the knob is held to 

 the prime conductor of an 

 electrical machine, the outer 

 coating being either held in 

 the hand or connected to " earth " by a wire or chain. 

 When a change of + electricity is imparted thus to the 

 inner coating, it acts inductively on the outer coating, 

 attracting a charge into the face of the outer coating 

 nearest the glass, and repelling a + charge to the outside 

 of the outer coating, and thence through the hand or wire 

 to earth. After a few moments the 

 jar will have acquired its full charge, 

 the outer coating being - and the 

 inner + . If the jar is of good glass, 

 and dry, and free from dust, it will 

 retain its charge for many hours or 

 days. But if a path be provided by 

 which the two mutually attracting 

 electricities can flow to one another, 

 they will do so, and the jar will be 

 instantaneously discharged. If the 

 outer coating be grasped with one 

 hand, and the knuckle of the other 

 hand be presented to the knob of the jar, a bright 

 spark will pass between the knob and the knuckle 

 with a sharp report, and at the same moment a convulsive 



