84 ELEMENTS OF STATIC ELECTRICITY. 



It should also be noticed that when the leaves are 

 held loosely, the thickness of the air dielectric is in- 

 creased ; each layer of air having a charged surface of 

 partly conducting paper on each side of it, is in the 

 position of the coated pane, a powerful attraction 

 between the surfaces acting across it. And when the 

 paper bursts there is more room for the formation of a 

 burr, and less resistance to the tearing of the paper, 

 which accounts for the increased prominence of the 

 burr. 



If the charge of the jar is negative, the same results 

 occur in reverse order. 



THE RESIDUAL CHARGE. When a Leyden jar is 

 discharged, there still remains a slight difference of 

 potential between the coatings, which is known as the 

 residual charge. Hence, a small discharge can be 

 obtained a moment after the first ; and this also leaves 

 a residual, bearing about the same proportion to the 

 second discharge as the second to the first, when the 

 same length of time elapses between them. A number 

 of successive discharges may thus be obtained, which 

 constantly decrease in amount till no further discharge 

 is perceptible. But, even then, it is not probable that 

 perfect equilibrium is restored. 



To understand this, we must remember that even 

 the best dielectric is a partial conductor: and that 

 while electric movement is instantaneous in a good 

 conductor, it is very slow in a non-conductor. In the 

 Leyden jar we have a combination of both two con- 

 ductors separated by a non-conductor. And, when the 

 charge is given, every part of each coating instantly 

 becomes electrified, one coating positively and the other 

 negatively, on the surfaces next the glass. 



