Electric Residue in the Leyden Jar. 309 



disposable charge, that is to say, becomes changed into a dispo- 

 sable charge. 



This second portion, r, has been named the residue, and we 

 will retain the term ; and by the term concealed residue, we will 

 denote the said portion while it remains inefficient in the jar, 

 calling that portion which makes its appearance after the dis- 

 charge the restored residue. 



The observation of the restored residue is best accomplished 

 by means of the sine-electrometer, by connecting the knob of 

 the jar with the instrument immediately after the jar has been 

 discharged. It is at once seen that the needle commences to 

 diverge from the magnetic meridian ; and by following the needle 

 and reading off the deflections, we obtain a series of points suffi- 

 ciently near to each other to permit of a curve being constructed 

 which shall represent the augmentation of the charge. The 

 ordinates of this curve increase most speedily at the commence- 

 ment, so that in this case also the curve is not unlike a parabola : 

 but the greater the loss of electricity in the air, the sooner does 

 a point arrive where this increase passes over into a diminution ; 

 this point represents the moment when the exhibition of the 

 residue is exactly balanced by the loss in the air. When the 

 lattei*, on account of the nature of the jar, is very small, the 

 augmentation continues for several hours; and for hours the 

 charge appears to remain perfectly unchanged, the discharge 

 requiring a number of days to effect it completely. AVhen, how- 

 ever, the quantity lost in the air, or on account of imperfect in- 

 sulation, is considerable as compared with the size of the jar, the 

 curve, which ascends for a short time abruptl)^, is suddenly bent 

 downwards. Drawings of such curves are given in fig. 2 p. 



When the first residue has been discharged, a second small 

 residual curve is observed, which proves that a concealed residue 

 was still remaining ; this can be repeated several times. 



This repetition of the discharge is the only means of learning 

 how much of a chai'ge has changed itself into a concealed residue r. 

 To reduce the loss of electricity during the process to a minimum, 

 a magnetic needle of very small magnetic moment is necessary ; 

 and the observations must be made with small angles between 

 the needle and the repelling arm. We first pei'mit as much 

 of the residue to return as can be observed with certainty, then 

 discharge*, read off the angle, and repeat the process as long 

 as a measurable residue remains. Although this process fre- 

 quently continues for several hours, still, in comparison with the 

 total residue, the loss of electricity is very small. The greater por- 

 tion of the residue appears very quickly at the commencement of 



* It is essential here that the outer coating is perfectly discharged, for 

 otherwise the deflection is illusory. 



