ELECTRICITY IN MOTION. 41 



rry 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. 



The immediate intensity of the electricity may be measured, and 

 its character distinguished, by electrical balances, and by electro- 

 meters 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 applying this force is by the torsion of 

 a wire, which has been employed for the purpose by Mr. Coulomb. 

 The quadrant electrometer of Henley expresses the mutual repul- 

 sion of a moveable ball and a fixed column, by the divisions of the 

 arch to which the ball rises. These divisions do not exactly de. 

 note the proportional strength <5f the action, but they are still of 

 utility in ascertaining the identity of any two charges, and in in. 

 forming us how far we may venture to proceed in our experiments 

 with safety ; and the same purpose is answered, in a manner some- 

 what 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 electrometer is still more de. 

 licate ; it consists of two small portions of gold leaf, suspended 

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

 nicated by contact : a very weak electricity is sufficient to make 

 them diverge, and it may easily be ascertained whether it is posi- 

 tive 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, prevents the 

 communication of any electricity to the glass, which might inter- 

 fere with the action of the instrument. When the balls of an 

 electrometer stand at the distance of four degrees, they appear to 

 indicate a charge nearly eight times as great as when they stand at 

 one degree : a charge eight times as great in each ball, producing a 

 mutual action sixty. four times as great at any given distance, and 



