268 M. Becquerel on the Development 



of the circumference of a divided circle traced on the hori- 

 zontal shelf. 



The circumstance of the case being more or less cylindrical, 

 can in no respect alter the value of the degrees. The position 

 of the arm of the lever which carries the disk of gilt paper, is 

 exactly determined by means of a vertical rule placed on a 

 circular foot, which is passed along the circumference of the 

 circle, and whose centre corresponds to the prolongation of the 

 •wire of torsion. 



Let us now see the effect of the equal division of electricity 

 between the two disks. It is well known that the total force 

 of the repulsion varies, at each distance, in the same ratio as 

 the quantities of electricity which contribute to this repulsion : 

 it follows therefore that the expression of its force, which is 

 called Electric Reaction, must be in proportion to the product 

 of these two quantities. Now let x represent the excess of 

 electricity acquired by the disk of cork or of elder-pith at its 

 escape from compression ; immediately after contact with the 

 disk of gilt paper both will possess an excess of electricity \x ; 

 the electric reaction will then be expressed by j x- : this same 

 reaction is however given directly by experiment, since the arc 

 of the circle which measures the distance of the two disks is 

 in proportion to it, in so far as that arc may be taken for its 

 chord ; which may always be done by twisting sufficiently the 

 suspending wire. _ 



Let e be this arc, we shall have e= \ x'^; whence x = 2V e. 

 This is the value of the quantity of electricity acquired by the 

 disk of elder-pith after a pressure P. For another pressure P' 

 we shall have a' = 2 */ e' ; but if in the two experiments the two 

 disks are brought back to the same distance by means of tor- 

 sion, the values of x and x^ will become comparable, and we 

 shall deduce from them the ratio of the electric intensities at- 

 tributable to different pressures. 



This manner of determining the quantity of electricity pro- 

 duced by any pressure whatever, may be employed whenever the 

 repulsion of the disk of gilt paper, after the division of the elec- 

 tricity, is measured by an arc of a certain number of degrees ; 

 but the development of electricity is frequently so feeble that 

 the electric reaction becomes unappreciable, notwithstanding 

 the great sensibility of torsion of the platinum wire. We must 

 therefore have recourse to another expedient: for this pur- 

 pose, it is sufficieat to give to the disk of gilt paper a quantity 

 of electricity whose intensity may be determined at every expe- 

 riment; all the results will then become comparable ; a second 

 disk of gilt paper of the same diameter as the first, and in- 

 sulated 



