ON ELECTRICITY IN MOTION. 527 



degree of tension ; and when the two plates of this condenser are sepa- 

 rated, they both exhibit an electricity much more powerful than that of 

 the first condenser. The force is, however, still more rapidly augmented 

 by the instruments of Mr. Bennet* and Mr. Nicholson,t although it has 

 been supposed that these instruments are more liable to inconvenience 

 from the attachment of a greater portion of electricity to the first plate 

 of the instrument, which leaves, for a very considerable time, a certain 

 quantity of the charge not easily separable from it. Mr. Bennet employs 

 three varnished plates laid on each other, but Mr. Nicholson has substituted 

 simple metallic plates, approaching only very near together, so that there 

 can be no error from any accidental friction. In both of these instruments, 

 the second plate of a condenser acquires an electricity contrary and nearly 

 equal to that of the first, by means of which it brings a third plate very 

 nearly into the same state with the first ; and when the first and third 

 plates are connected and insulated, they produce a charge nearly twice as 

 great in the second plate, while the first plate becomes at the same time 

 doubly charged ; so that by each repetition of this process, the intensity of 

 the electricity is nearly doubled : it is therefore scarcely possible that any 

 quantity should be so small as to escape detection by its operation. (Plate 

 XL. Fig. 562, 563.) 



The immediate intensity of the electricity may be measured, and its 

 character distinguished, by electrical balances, and by electrometers of 

 different constructions. The electrical balance measures the attraction or 

 repulsion exerted by two balls at a given distance, by the magnitude of the 

 force required to counteract it ; and the most convenient manner of apply- 

 ing this force is by the torsion of a wire, which has been employed for the 

 purpose by Mr. Coulomb.^ The quadrant electrometer of Henley ex- 

 presses the mutual repulsion of a moveable ball and a fixed column, by the 

 divisions of the arch to which the ball rises. These divisions do not 

 exactly denote the proportional strength of the action, but they are still of 

 utility in ascertaining the identity of any two charges, and in informing us 

 how far we may venture to proceed in our experiments with safety; and 

 the same purpose is answered, in a manner somewhat less accurate, by the 

 electrometer, consisting of two pith balls, or of two straws, which are 

 made to diverge by a smaller degree of electricity. Mr. Bennet's electro- 

 meter || is still more delicate ; it consists of two small portions of gold leaf, 

 suspended from a plate, to which the electricity of any substance is com- 

 municated by contact : a ve*y weak electricity is sufficient to make them 

 diverge, and it may easily be ascertained whether it is positive or negative, 

 by bringing an excited stick of sealing wax near the plate, since its 

 approach tends to produce by induction a state of negative electricity in 

 the remoter extremities of the leaves, so that their divergence is either 

 increased or diminished, accordingly as it was derived from negative or 

 from positive electricity : a strip of gold leaf or tin foil, fixed within the 



* Ph. Tr. 1787, pp. 32, 288 ; 1794, p. 266. 

 t Ibid. 1788, p. 403. Nich. Jour. i. 395 ; ii. 370 ; iv. 95. 

 % Hist, et Mem. 1785, p. 569. Ph. Tr. 1772, p. 359. 



1 Ph. Tr. 1787, p. 26. 



