ELECTFICITY. 



who holds it, to the ground. If, instead 

 of this, the jar be placed on an insulat- 

 ing stand, and a ball of metal, or the 

 knuckle of the finger, be held near the 

 outside of the jar, \ve have evidence of 

 the escape of the electricity from the 

 latter by a succession of sparks simul- 

 taneous with those that occur between 

 the prime conductor and the knob of 

 the jar. 



(126.) If, instead of touching the 

 outer coating of a jar supported on an 

 insulating stand, we bring into contact 

 with it the knob of a second jar, of 

 which the outer coating communicates 

 with the ground, as shewn in jig. 31, 



Fig. 31. 



the electricity which is expelled from 

 the outer coating of the first jar passes 

 into the inner coating of the second jar, 

 and thus both jars are charged. Thus 

 may charges be given to a succession 

 of jars, so placed as that the inner 

 coating of each shall communicate with 

 the outer coating of the one that pre- 

 cedes it in the series ; taking care that 

 the outer coating of the last jar com- 

 municates with the ground. All the 

 jars will be found to be charged in a 

 similar manner. It is evident, however, 

 that the charge must diminish in in- 

 tensity as it is conveyed from each jar 

 to the next, because the quantity of 

 electricity which is expelled from the 

 exterior is never quite equal to that 

 which passes into the interior. 



(127.) For the sake of greater dis- 

 tinctness we have ah 1 along supposed the 

 interior of the jar to be charged with 

 positive electricity, but the very same 

 effect would take place if the knob of 

 the jar were charged negatively by com- 

 munication with the negative conductor. 

 A similar change in the electrical state 

 of the coatings would result from placing 

 the jar on an insulating stand, and then 



forming a "communication between the 

 outer coating and the prime conductor, 

 while the knob is made to communicate 

 with the ground. The only (iiiiercnce 

 is, that the outer coating would then 

 be active and the inner one neutral; 

 but these conditions would again be 

 reversed as soon as the knob was dis- 

 connected with the ground, and the 

 outer coating touched with the hand. 



(128.) If two jars, the one charged 

 positively, the other negatively, be 

 placed on two separate insulating 

 stands, and their knobs then connected 

 by a conduct or, vrhich is itself insulated, 

 no explosion will take place, although 

 the two coatings, which are thus brought 

 into communication, are in opposite 

 electrical states. But if the two outer 

 coatings be at the same time connected, 

 an explosion will take place, and both 

 jars will be discharged. 



(129.) Since the susceptibility of re- 

 ceiving a charge depends upon the 

 proximity of the metallic surfaces, while 

 the passage of the electricity from the 

 one to the other is interrupted by the 

 interposition of a non-conducting sub- 

 stance, it is evident that, in the con- 

 struction of the Leyden j ar, the thick- 

 ness of the glass is an important con- 

 sideration. The thinner the glass, the 

 greater.. will be the power of taking a 

 charge ; but the power of retaining the 

 charge will be less, on account of the 

 diminished resistance which the glass 

 will afford to the passage of the elec- 

 tricity through it. If the charge be 

 higher than what the jar will bear, the 

 glass will be broken by the violence with 

 which the electricity forces a passage 

 through its substance. Muscovy talc, 

 even in very thin laminae, resists much 

 better than glass, and is, therefore, ca- 

 pable of receiving and of retaining a 

 much higher charge. Another limit 

 to the charge which a jar is capable of 

 retaining, arises from the liability of the 

 electricity to pass from one coating to 

 the other, round the edges of the glass. 

 (130.) These spontaneous discharges, 

 as they are called, are facilitated by the 

 deposition of moisture on the glass, 

 forming a chain of conducting particles 

 in the very line which the electricity 

 has a strong tendency to take. Hence, 

 it is a requisite precaution to keep the 

 apparatus in as dry a state as possible ; 

 and the deposition of moisture may be 

 guarded against most effectually by 

 covering the uncoated part of the glass 

 with a layer of sealing-wax, or other 

 D 2 



