Methods of making the comparisons 153 



296] In making the experiment one of the plates whose charges we 

 want to compare together, or the plate B as we will call it, is laid on the 

 bars Nn and Pp, between the sticks of glass and end N, the upper coating 

 thereof being made to communicate with Bb and Mm by a wire V resting 

 on Mm, and the lower coating is made to communicate with the ground 

 by a springing wire 5 fastened to Rr, and by its elasticity bearing against 

 the lower coating of the plate. 



Another coated plate is laid on the same bars between the sticks of 

 glass and n by way of trial plate, the upper coating of which communicates 

 with Aa by the wire /?, and the lower coating communicates with Dd by 

 the springing wire 8. A pair of pith balls also, such as were used in the 

 former experiments, were suspended from D as represented in the figure. 



In trying the experiments, the jars, and consequently the wire Cc, 

 are charged, the wire Cc being all that time lifted up as high as it will 

 go by means of the counterpoise. When the jars are charged to the proper 

 degree as shewn by the electrometer, the wire Cc is let down on the wires 

 A a and Bb by lifting up the counterpoise. This instantly charges both 

 the coated plates, for when Cc rests on A a and Bb, and consequently Ff 

 and Gg rest on Ee and Dd, the lower coatings of both plates communicate 

 with the ground, and their upper coatings with Cc. 



Immediately after this the counterpoise is let go, by which means Cc 

 is lifted up, and Gg and Ff along with it, till the two last mentioned wires 

 bear against Aa and Bb, so that immediately after the coated plates are 

 charged, the communication between them and the wire Cc, by which 

 they were electrified, is taken away, and at the same time the communica- 

 tion between the lower coating of the trial plate and the ground is taken 

 away, and immediately after that a communication is made between the 

 upper coating of the plate B and the lower coating of the trial plate, and 

 also a communication is made between the upper surface of the trial plate 

 and the ground, so that the upper coating of the trial plate and the lower 

 coating of the plate B both communicate with the ground, and the upper 

 coating of B and the lower coating of the trial plate communicate with 

 each other and the wire Dd. 



Consequently, if the quantity of redundant fluid communicated to the 

 wires Bb and Mm and the upper side of the plate B together is equal to 

 the deficient fluid on the under side of the trial plate, they and the wire 

 Dd will be neither over nor undercharged after the operation is completed; 

 but if the redundant fluid in them exceeds the deficient fluid on the lower 

 side of the trial plate, Dd will be overcharged, and the pith balls will 

 separate positively. On the other hand, if it is less than the deficient fluid, 

 the pith balls will separate negatively. 



297] The trial plate consisted of a flat plate of glass, or other electric 

 substance, the lower surface of which was coated all over with tinfoil, 



