ELECTRICITY. 



52 



on the cake : in this situation it does 

 not come sufficiently in contact with 

 the cake to receive its electricity, but 

 acquires by induction an opposite state 

 at its lower surface, and a similar state 

 at its upper; that is, the cake being 

 negative, the under side of the cover 

 will be positive, and the upper side 

 negative. If, while in this state, the 

 upper negative surface be touched with 

 the finger, or with any other conductor 

 communicating with the earth, a spark 

 will pass from the latter to the cover, 

 so as to restore the electric equilibrium ; 

 the quantity of electricity thus super- 

 added being retained in the cover by 

 the inductive influence of the cake. 

 But when the plate is raised, provided 

 it be held by its insulating handle, the 

 action of the cake being withdrawn, the 

 cover is found to be charged with posi- 

 tive electricity, which may be imparted 

 to an insulated conductor, or to a 

 Leyden jar. This operation maybe re- 

 peated an indefinite number of times, 

 since the electricity of the cake con- 

 tinues unimpaired during the process, 

 and thus may a charge be communi- 

 cated to the jar of an intensity equal to 

 that of the cover of the electrophorus 

 when raised. The instrument has been 

 known, indeed, to retain its power un- 

 diminished for months, and may there- 

 fore be regarded as a sort of magazine 

 of electricity. It is obvious, that if the 

 cover were simply placed on the cake, 

 and again raised without previously 

 touching it, it would then exhibit no 

 sign of electricity. If the sole of the 

 electrophorus be insulated, a spark may 

 be obtained from it, when the cake 

 has been excited ; and if while placed 

 on the cake the cover be touched with 

 the finger, and at the same time the 

 sole be touched with the thumb, a sen- 

 sible shock will be felt in that part of 

 the hand. 



(193.) Volta is also the inventor of 

 an instrument acting on the same prin- 

 ciple as the electrophorus, and which he 

 termed the condenser, of which the 

 purpose is to collect a weak electricity, 

 spread over a large surface, into a body 

 of small dimensions, in which its in- 

 tensity will be proportionably increased, 

 and therefore become capable of being 

 examined. A small metallic plate, 

 connected with the substance of which 

 the electricity is to be determined, is 

 brought within a very small distance of 

 another plate communicating with the 

 earth. The small portion of electricity 



received from the substance to be tried 

 by the first plate, acts by induction on 

 the second plate, and occasions it to 

 acquire the opposite electrical state: 

 this latter state reacts upon the first 

 plate, increasing its capacity for the 

 electricity which it had first received, and 

 tends to accumulate a larger quantity 

 in it, which quantity it must derive from 

 the substance with which it communi- 

 cates. This mutual action and reaction 

 continues till an equilibrium is attained. 

 If the communication between the sub- 

 stance tried and the first plate be 

 broken off, and the plate thus insulated 

 be removed from the contiguity of the 

 second plate, the accumulated electricity 

 with which it is charged will become 

 evident upon its application to an or- 

 dinary electroscope, such as those de- 

 scribed in 13 and 14. 



(194.) Various have been the forms 

 given to the condenser, according to 

 the fancy of electricians, without any 

 change in the principle on which it 

 acts. In general, the two plates are 

 merely separated by a thin stratum of 

 air. Sometimes their surfaces are 

 covered with a non-conducting varnish, 

 which prevents any communication of 

 electricity from the one plate to the 

 other, while it allows of a very near ap- 

 proach of the plates to each other ; but 

 this method is liable to objection, from 

 the permanent electricity which the 

 varnish sometimes contracts by friction, 

 and which may interfere with the re- 

 gular operation of the instrument. One 

 of the most convenient forms is that of 

 the condensing electrometer, (fig. 40,) 

 Fig. 40. 



in which the first plate of the condenser 

 A, is fixed to the cap of the gold-leaf 

 electroscope; the second plate B, which 

 communicates by a chain with the 

 ground, being moveable round a joint 

 C, and thus capable of being turned 

 back and removed from the first plate, 

 so as to allow its electricity to be ma- 

 nifested by the divergence of the gold 

 leaves. 



