ON ELECTRICITV IN MOTION. 683 



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, Avhich are made to diverge by a smaller degree of 

 electricity. Mr, Eennet's electrometer is still more delicate; it consists of 

 two small portions of gold leaf, suspended from a plate, to whicli the electri- 

 city of any substance is communicated by contact: a very 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 glass 

 which covers the electrometer, opposite to the extremities of the leaves, pre- 

 vents the communication of any electricity to the glass, which might interfere 

 with the action of the instrument. When the balls of an electrometer stand 

 at the distance of 4 degrees, they appear to indicate a charge nearly 8 times 

 as great as m hen they stand at one degree : a charge 8 times as great in each 

 ball producing a mutual action 64 times as great at any given distance, and 

 at a quadruple distance a quadruple force; in the same manner a separation of 

 9 degrees is probably derived from an intensity 27 times as great as at 1. In 

 Lane's electrometer the magnitude of a shock is determined by the quantity 

 of air through which it is obliged to pass, between two balls, of which the 

 distance may be varied at pleasure; and the power of the machine may be 

 estimated by the frequency of the sparks which pass at any given distance. 

 It appears from Mr. Lane's experiments, that the quantity of electricity re- 

 quired for a discharge is simply as the distance of the surfaces of the balls, 

 the shocks being twice as frequent when this distance is only ^ of an inch 

 as when it is -^. Mr. Volta says, that the indications of Lane's and 

 Henley's electrometer agree immediately with each other; but it seems diffi- 

 cult to reconcile this result with the general theory. Sometimes the force of 

 repulsion between two balls in contact is opposed by a counterpoise of given 

 magnitude, and as soon as this is overcome, they separate and form a circuit 

 which discharges a battery; whence the instrument is called a discharger. 

 (Plate XL. Fig. 564 . . 568. ) 



It must be confessed that the whole science of electricity is yet in a very 



