of JLlcctricity hij Pressure. 267 



beam of the balance rises and falls at will by means of a 

 screw. The apparatus is then placed in such a manner, 

 that the beam may be in a horizontal direction when the two 

 bodies subjected to pressure just touch each other. Things 

 being thus arranged, the pressure is applied ; after which, in 

 order to withdraw the bodies from pressure, two springs are 

 used, disposed in such a manner as to receive a determinate 

 tension. The springs, returning to their original state, draw 

 with them the beam ; the bodies are therefore withdrawn from 

 compression with a velocity equal to the tension of the springs. 

 The body placed at the extremity of the tube being with- 

 drawn from compression, it is necessary to present it to the disk 

 of gilt paper in the direction of its greatest surface ; which is 

 accomplished by forming this tube of two pieces and joining 

 them by means of bolting joints (charniere a boulon) : then 

 inserting this tube, which is solid, into another a little larger, 

 the two parts are in a right line. If it is desired to present the 

 body to the disk of gilt paper, the smaller tube is drawii a little 

 way out of the one in which it is inclosed, and the upper 

 part immediately slips down. 



Most frequently the excess of electricity acquired by each 

 of the substances, on the cessation of the pressure, is very 

 small ; consequently, if a silver wire like that used by Coulomtj 

 in his experiments be employed as the wire of torsion, the 

 repulsion would be scarcely sensible, and sometimes null ; 

 we must therefore substitute another whose force of torsion 

 is much weaker. Very fine platinum wire drawn according to 

 Dr. WoUaston's plan, perfectly answers this purpose : but 

 care must be taken to choose the proper degree of fineness ; 

 for I have ascertained that when these wires have attained a 

 certain degree of tenuity, the torsion of a small angle deranges 

 the aggregation of the molecules so much, that they do not 

 recover their jirimitive position of equilibrium. The result is, 

 that the oscillations of a horizontal pendulum suspended to 

 the extremities of this wire are no longer isochi'onous ; it must 

 therefore be I'ejected. 



One of the bodies is placed between the two copper plates ; 

 jjolished on one of its surfaces, if it is mineral and suscep- 

 tible of being polished ; the other, for which cork or elder- 

 pith is usually taken in comparative experiments, is in the form 

 of a small disk of very little thickness, and of the same diame- 

 ter as the disk of gilt paper of the balance. It follows, that when 

 these two disks, after liaving both been brought to zero, are 

 in contact, they possess an ccjual share of electricity: repulsion 

 immediately takes place, and the effect is measured by means 



L 1 2 'of 



