116 



ELECTRICITY. 



he takes from it nearly all the electricity with which it is charged over and 

 above its natural amount ; for his body being of so much greater magnitude 

 thnn the tube, the proportion which will remain on the tube will be insig- 

 nificant. 



s If when A. nibs the tube he were not insulated, he would not be electrified, 

 because, as fast as his body would lose its proper amount of electricity, the 

 deficiency would be made up from the earth, with which he is in free electri- 

 cal communication ; whereas in the former case being insulated, this supply 

 could not be obtained. Hence, in this theory, the earth is regarded as the 

 common reservoir of electricity, from which bodies negatively electrified re- 

 ceive what they want, and to which bodies positively electrified give up their 

 surplus, except in the case in which the one or the other are insulated. 



Such, in general, was the Franklinian theory; which, however, will be 

 more fully developed hereafter. 



Assuming these hypothetical principles, Franklin next proceeded to analyze 

 the phenomena of the Leyden jar. His first experiments were directed to es- 

 tablish the fact, that when the jar is charged, the inside is electrified positively, 

 and the outside negatively. A charged jar was placed on an insulating sup- 

 port, and a metallic wire bent into the form of a circular arc was then placed 

 with one end in contact with the outer coating. The other end was capable 

 of being brought into contact with the hook of the wire inserted through the 

 cork, and thereby put in metallic communication with the water contained in 

 the jar. This bent wire being supported by a handle of sealing-wax was 

 itself insulated, and no electricity could pass in the experiment, otherwise than 

 between the inside of the jar and the coating on the outside. On bringing the 

 upper extremity of the bent wire into contact with the hook, the jar was in- 

 stantly discharged, both the inside and the outside being restored to their 

 natural state. Franklin inferred from this, that before the discharge the in- 

 terior of the jar was positively electrified, and the exterior coating negatively 

 electrified, in an equal degree ; that is to say, that the interior of the jar con- 

 tained an excess of electricity over and above its natural amount, and the ex- 

 terior coating fell short of its natural amount by a quantity equal to that excess. 



Various other experiments were made to verify this doctrine. Two metallic 

 knobs were placed near each other, one communicating with the external 

 coating, and the other with the water within the jar. A small cork ball sus- 

 pended by a silk thread was placed between those two knobs. The ball was 

 alternately attracted and repelled, " playing incessantly from one to the other, 

 the bottle was no longer electrised ; that is, it fetched and carried fire from 

 the top (inside) to the bottom (outside) of the bottle, till equilibrium was re- 

 stored."* 



i had been observed by electricians in Europe, that a jar could not be 

 charged if its external coating were insulated ; that, in fact, it was a necessary 

 condition that a communication between that coating and the ground should be 

 provided and maintaiaed by some conducting matter, such as a metallic wire. 

 Franklin assumes, that no electricity can be conveyed to the inside without 

 causing the expulsion of an equal quantity from the outside ; but if the jar be 

 insulated, no means of escape being left for the electricity on the outside, no 

 accumulation can take place on the inside. f 



In these experiments, we find also a description of the method of charging 

 a series of jars, now called the charge by cascade. " Suspend two or more 

 phials on the prime conductor, one hanging on the tail of another, and a wire 

 irom the last to the floor. An equal number of turns of the wheel will charge . 

 them all equally, and every one as much as one alone would have been ; what i 



* Franklin's Works (Letters), vol. v., p. 192. Boston. 1837. t Letters, p. 190. 



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