682 LECTORE LIV. 



JNfr. Cavallo's multiplier 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 ex- 

 amined, to the first or insulated plate of the second condenser, which receives 

 it repeatedly, until it has acquired an equal degree of tension; and when the 

 two plates of this condenser are separated, 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, although it has been supposed that these instruments are more 

 liable to inconvenience from the attachment of a greater portion of electri- 

 city 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. Ben- 

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

 substituted simple metallic plates, approachingonly very near together, so that 

 there can be no error from any accidental friction. In both of these instru- 

 ments, 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. 56<2, 563.) 



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

 racter distinguished, by electrical balances, and by electrometers of difierent 

 constructions. The electrical balance measures the attraction or repulsion 

 exerted by two balls at a given distance, by tlie magnitude of the force re- 

 quired 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 

 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 propor- 

 tional 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 



