ELECTRICAL APPARATUS. 159 



and size of other conductors in its neighbourhood. The effect 

 of all these conditions is expressed by the term capacity, the 

 capacity of a conductor being the quantity of electricity that is 

 required to change its electric potential by unity. Hence, in 

 general terms, 



the quantity of electricity in a conductor =: its electrical potential X 

 its electrical capacity. 



It follows that, when a conductor has been charged up to the 

 attainable limit of potential, the amount of the charge can be 

 increased only by increasing its capacity, or, in other words, the 

 ratio of the quantity to the potential of the charge. The most 

 effectual way of doing this is to place another conductor near the 

 one that is to be charged, and to give it a charge of the opposite 

 kind to that of the latter. This is done in the instruments known 

 as ELECTRICAL CONDENSERS and ACCUMULATORS, of which the 

 Leyden jar is -the most familiar example. In this apparatus the 

 conductor to be charged is a sheet of tinfoil pasted on glass, 

 a second piece of tinfoil being pasted opposite to it on the other 

 side of the glass. To charge the first sheet, say positively, it is 

 connected with the prime conductor of an electrical machine, 

 and the second sheet of tinfoil is connected (either directly, or 

 through the earth) with the rubber of the machine : or, if any other 

 source of electricity is used, the two sheets of tinfoil or "coat- 

 ings," as they are usually called are connected with the parts 

 which correspond with the conductor and rubber respectively. 

 The negative electrification of the second coating then diminishes 

 the positive potential due to the positive charge of the first coating, 

 and vice versa, so that when the full difference of potential, 

 which the machine employed is capable of producing, has been 

 established between the coatings, the quantity of electricity accu- 

 mulated in each of them is many times as great as that which it 

 would have received in the absence of the other. 



When the limit of the charge which can be given to a conductor 



