ELECTRICITY IN MOTION. S9 



may contain within it a jar, which may be charged at once by the 

 operation of the machine, when its internal surface is connected 

 cither with the earfh, or with that of the jar contained in the op. 

 posite conductor. The glass may be either in the form of a cir- 

 cular plate or of a cylinder, and it is uncertain which of the ar- 

 rangements affords the greatest quantity of electricity from the 

 same surface ; but the cylinder is cheaper than the plate, and less 

 liable to accidents, and appears to be at least equally powerful. 



The plate machine in the Te\ lerian museum, employed by Van 

 Marum, when worked by two men, excited an electricity, of 

 which the attraction was sensible at the distance of thirty-eight 

 feet, and which made a point luminous at twenty. seven feet, and 

 afforded sparks nearly twenty-four inches long. Mr. Wilson had 

 also a few years ago, in the Pantheon in London, an apparatus 

 of singular extent; the principal conductor was 150 feet long, 

 and sixteen inches in diameter, and he employed a circuit of 4800 

 feet of wire. 



The electrophorus derives its operation from the properties of 

 induced electricity. A cake of a nonconducting substance, com. 

 monly of resin or of sulphur, is first excited by friction, and be. 

 comes negatively electric : an insulated plate of a conducting sub. 

 stance, being placed on it, does not come sufficiently into contact 

 with it to receive its electricity, but acquires by induction an op- 

 posite state at its lower surface, and a similar state at its upper; 

 so that when this upper and negative surface is touched by a sub. 

 stance communicating with the earth, it receives enough of the 

 electric fluid to restore the equilibrium. The plate then being 

 raised, the action of the cake no longer continues, and the elec- 

 tricity, which the plate has received from the earth, is imparted 

 to a conductor or to a jar; and the operation may be continually 

 repeated, until the jar has received a charge, of an intensity equal 

 to that of the plate when raised. Although the quantity of elec- 

 tricity received by the plate, is exactly equal to that which is 

 emitted from it at each alternation, yet the spark is far less 

 sensible ; since the effect of the neighbourhood of the cake is to 

 increase the capacity of the plate, while the tension or force im- 

 pelling the fluid is but weak ; and at the same time the quantity 

 received is sufficient, when the capacity of the plate is again dimi- 

 nished, to produce a or ucb greater tension, at a distance from 

 the cake 



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