526 LECTURE LIV. 



also a few years ago, in the Pantheon in London, an apparatus of singular 

 extent; the principal conductor was 150 feet long, and 16 inches in 

 diameter, and he employed a circuit of 4,800 feet of wire.* 



The electrophorus derives its operation from the properties of induced 

 electricity. A cake of a nonconducting substance, commonly of resin or of 

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

 insulated 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 

 substance 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 a jar ; 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, received 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 neighbourhood 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 received is sufficient, when 

 the capacity of the plate is again diminished, to produce 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 

 electrophorus, both instruments deriving their properties from the effects of 

 induction. 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 general a thin stratum of air only is interposed ; but sometimes 

 a nonconducting varnish is employed ; this method is, however, liable to 

 some uncertainty, from the permanent electricity which the varnish some- 

 times contracts by friction. The electricity is accumulated by the attrac- 

 tion of the plate communicating 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 sub- 

 stance, 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 facilitated by 

 several subordinate arrangements. (Plate XL. Fig. 561.) 



Mr. Cavallo's multiplierf is a combination of two condensers ; the 

 second or auxiliary plate of the first, like the plate of the electrophorus, is 

 moveable, and carries a charge of electricity, contrary to that of the 

 substance to be examined, to the first or insulated plate of the second 

 condenser, which receives it repeatedly, until it has acquired an equal 



* Wilson's Account of his Experiments, 4to, 1778. \ 



t Nich. Jour. i. 394. 



