ELECTRICITY. 



23 



positive (vitreous) electricity, by friction against the cushion, and is as 

 quickly discharged, by the row of points, into the prime conductor, 

 which, as it is insulated, thus acquires a charge of positive elec- 



Fig. 7. 



Fig. 8. 



tricity, which it will yield up on contact with any body, with a 

 peculiar noise and spark. The maximum effect is produced when 

 the rubber is made to communicate with the earth by means of a 

 chain. If negative electricity be wanted, the conductor must be 

 made to communicate with the earth, and the rubber insulated. 



The Leyden jar is an instrument for accumulating electricity. 

 It depends on the principle that a large amount of the 

 two different sorts of electricity may reside on the 

 two surfaces without any tendency to equilibrium, on 

 account of the non-conducting power of the glass. 

 It consists of a thin glass jar coated on both sides 

 with tin foil to within a few inches of the top ; a 

 wire, terminating in a metallic knob, communicates 

 with the interior coating. When the outside coating 

 is connected with the earth, and the knob placed 

 near the prime conductor of the machine, the inner 

 and outer surfaces become respectively positive 

 and negative. If, now, the two coatings be made to 

 connect by means of a bent wire, the equilibrium is 

 restored, a bright spark is perceived, along with a sharp snap, and 

 if the body be interposed, the electric shock is felt. The electric bat- 

 tery is only a great number of such jars connected together by their 

 inner and outer coatings respectively ; the whole then act as one 



