ON ELECTRICITY IN MOTION. 681 



sulfur, is first excited by friction, and becomes negatively electric: an in- 

 sulated plate of a conducting substance, being placed on it, does not come 

 sufficiently into contact with it to receive its electricity, but acquires by 

 induction an opposite state at its lower surface, and a similar state at its 

 upper; so that when this upper and negative surface is touched by a sub- 

 stance communicating with the earth, it receives enough of the electric fluid 

 to restore the equilibrium. The plate then being raised, the action of the 

 cake no longer continues, and the electricity, which the plate has received from 

 the earth, is imparted to a conductor or to ajar; and the operation may be 

 continually repeated, until the jar has received a charge, of an intensity equal 

 to that of the plate when raised. Although the quantity of electricity, re- 

 ceived by the plate, is exactly equal to that which is emitted from it at each 

 alternation, yet the spark is far less sensible; since the effect of the neighbour- 

 hood of the cake is to increase the capacity of the plate, while the tension or 

 force impelling the fluid is but weak; and at the same time the quantity re- 

 ceived is sufficient, when the capacity of the plate is again diminished, to pro- 

 duce a much greater tension, at a distance from the cake. (Plate XL. 

 Fig. 560.) 



The condenser acts in some measure on the same principles with the elec- 

 trophorus, both instruments deriving their properties from the effects of induc- 

 tion. The use of the condenser is to collect a weak electricity from a large 

 substance into a smaller one, so as to make its density or tension sufficient to 

 be examined. A small plate, connected with the substance, is brought nearly 

 into contact with another plate communicating with the earth; in gene- 

 ral a thin stratum of air only is interposed ; but sometimes a nonconducting 

 varnish is employed ; this method is, however, liable to some uncertaiqty, 

 from the permanent electricity which the varnish sometimes contracts by fric- 

 tion. The electricity is accumulated by the attraction of the plate communi- 

 cating with the earth, into the plate of the condenser; and when this plate 

 is first separated from the substance to be examined, and then removed from 

 the opposite plate, its electricity is always of the same kind with that which 

 originally existed in the substance, but its tension is so much increased as to 

 render it more easily discoverable. This principle has been variously applied 

 by different electricians, and the employment of the instrument has been fa- 

 ciUtated by several subordinate arrangements. (Plate XL. Fig. 561.)' 



