of Electricity by Pressure. 269 



sulated in the same manner, is placed in the case of the ba- 

 lance ; it is electrified by means of a brass wire which crosses 

 the case : the two disks of gilt paper are then brought into 

 contact by turning the screw which carries the micrometer ; an 

 equal division of electricity immediately takes place, and is fol- 

 lowed by repulsion. The micrometer is again turned to bring 

 back the two disks to a distance measured by a small arc, 

 one of ten degrees, for instance, which may be taken for its 

 chord. 



Let a be the number of degrees of torsion of the wire, 



V'10°+ a will represent the quantity of electricity possessed 

 by each disk of gilt paper. After making these arrangements, 

 let us withdraw the two bodies from compression, and place the 

 disk of cork, which is supposed to have acquired an excess of 

 electricity of the same nature as that communicated to the disk 

 of o-ilt paper, at zero of the horizontal circle, at the same time 

 brino-ino- back to it the disk of foil which possesses a quantity of 

 electricity -/ 10° +a; repulsion will take place. Let us bring 

 back the two disks to an angular distance of 10° by torsion, 

 and let d be the number of degrees of torsion to which we 

 must subject die wire for that purpose ; we shall then evidendy 

 have, representing by y. the excess of electricity possessed by 

 the disk of cork or elder-pith, 



yW 10 -\-a = \0 -^-d; whence, 

 _ lo + rf 



/y/ lO + a 



this is the expression of the quantity of electricity produced 

 by a pressure P. For another pressure P' we shall have 



'_ 10 + d^ 



y, a% d', being the quantities analogous to y, a, d. There- 

 fore, the electric intensities arising from die pressures P and 

 P' are in relation to each other as 



lO + d \0 + d' 



VlO + a ' VxO + a' ' 



Use of the Apparatus. 

 The surfaces of the bodies subjected to pressure ouglit to be 

 as nearly as possible in Uie same state of polish; witliout this 

 precaution, the electric results would not admit of comparison, 

 since the greater or less degree of poHsh has a remarkable in- 

 fluence on the (juantity of electricity developed. If they be 

 mineral substances, they should be cut into thin plates, and 

 brought to the highest degree of polish that art can attain; or, 

 wliich is preferable, ihoy may be cleaved or split naturally, 



when 



