ON ELECTRICITY IN EQUILIBRIUM. 513 



repulsive force, but when they are within a certain distance, the effects of 

 induced electricity overcome the repulsion which would necessarily take 

 place, if the distribution of the fluid remained unaltered by their mutual 

 influence. 



When a quantity of the electric fluid is accumulated on one side of a 

 non-conducting substance, it tends to drive off the fluid from the other 

 side ; and if this fluid is suffered to escape, the remaining matter exerts 

 its attraction on the fluid which has been imparted to the first side, and 

 allows it to be accumulated in a much greater quantity than could have 

 existed in an equal surface of a conducting substance. In this state, the 

 body is said to be charged ; and for producing it the more readily, each 

 surface is usually coated with a conducting substance, which serves to 

 convey the fluid to and from its different parts with convenience. The 

 thinner any substance is, the greater quantity of the fluid is required for 

 charging it in this manner, so as to produce a given tension, or tendency to 

 escape : but if it be made too thin, it will be liable to break, the attractive 

 force of the fluid for the matter on the opposite side overcoming the 

 cohesion of the substance, and perhaps forcing its way through the tem- 

 porary vacuum which is formed. 



When a communication is made in any manner by a conducting sub- 

 stance between the two coatings of a charged plate or vessel, the equili- 

 brium is restored, and the effect is called a shock. If the coatings be 

 removed, the plate will still remain charged, and it may be gradually dis- 

 charged by making a communication between its several parts in suc- 

 cession, but it cannot be discharged at once, for want of a common con- 

 nexion : so that the presence of the coating is not absolutely essential to 

 the charge and discharge of the opposite surfaces. Such a coated substance 

 is most usually employed in the form of a jar. Jars were formerly filled 

 with water, or with iron filings ; the instrument having been principally 

 made known from the experiments of Musschenbroek and others at 

 Leyden, it was called the Leyden phial ; but at present a coating of tin 

 foil is commonly applied on both sides of the jar, leaving a sufficient space 

 at its upper part, to avoid the spontaneous discharge, which would often 

 take place between the coatings, if they approached too near to each other ; 

 and a ball is fixed to the cover, which has a communication with the 

 internal coating, and by means of which the jar is charged, while the 

 external coating is allowed to communicate with the ground. A collection 

 of such jars is called a battery, and an apparatus of this kind may be 

 made so powerful, by increasing the number of jars, as to exhibit many 

 striking effects by the motion of the electric fluid, in its passage from one 

 to the other of the surfaces. 



The conducting powers of different substances are concerned, not only 

 in the facility with which the motions of the electric fluid are directed into 

 a particular channel, but also in many cases of its equilibrium, and par- 

 ticularly in the properties of charged substances, which depend on the 

 resistance opposed by nonconductors to the ready transmission of the fluid. 

 These powers may be compared, by ascertaining the greatest length of 

 of the substances to be examined, through which a spark or a shock 



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