ELECTftlCITV IN EaUILIBRIUM. 25 



for producing it the more readily, each surface is usually coated 

 with a conducting substance, which se rvs to convey the Quid to 

 and from its different parts with conveirence. 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 'he fluid ; for the matter ou the 

 opposite side overcoming the cohesion of the substance, and per. 

 haps forcing its way through the temporary vacuum which is 

 formed. 



When a communication is made in any manner, by a conducting 

 substance, between the two coatings of a charged plate or vessel, 

 the equilibrium is restored, and the effect is called a shock. If 

 the coatings be removed, the plate will still remain charged, and it 

 may be tiraHually discharged by making a communication between 

 its several parts in succession, but it cannot be discharged at once, 

 for want of a common connection : 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 coat, 

 ing of tin foil is commonly applied on both sides of the jar, leaving 

 a sufficient space at its upper part, to avoid the spontaneous dis- 

 charge, 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 particularly in the properties of charged sub. 

 stances, which depend on the resistance opposed by nonconductors 

 to the ready transmission of the fluid. These powers may be 



