612 ELECTRICAL CONDENSERS. 



trifled by drawing a rubbed glass rod once or several 

 times along it, or by connecting it with the conductor 

 of the machine. Whatever is added beyond the quantity 

 which the disc can hold is dissipated by its edge; even 

 the wire attached to the lower surface tends to increase 

 this dispersion. 



If a second similar disc, also provided with an insu- 

 lating handle of sealing-wax, B in fig. 322, be placed 

 upon the first, so that their surfaces are in perfect con- 

 tact, and an electrical charge be given to both, the 

 sparks are pot sensibly larger than those obtained from 

 one disc. The joint surface of both discs is not appre- 

 ciably greater than that of one ; hence the quantity of 

 electricity which can be accumulated in both is not 

 sensibly increased by the superposition of the two 

 discs. In order to investigate the electrical state of 

 either disc, let a pith ball be suspended by a linen or 

 cotton thread to the wire fixed in the upper disc, and 

 two similar pendulums to the lower disc, as indicated 

 at C in fig. 322. When both discs are electrified, the 

 upper pendulum is repelled by the wire from which it 

 is suspended, while the two pendulums below repel 

 each other. The divergence of the pendulums caused 

 by drawing the rubbed glass rod once along the discs is 

 not increased by drawing it several times along them ; 

 it follows that repeated electrical excitation does not 

 increase the quantity of electricity which the discs 

 retain. 



Now let a thin disc of some insulating substance, 

 such as ebonite, glass, or sealing-wax, having its 

 diameter about 2 cm greater than that of the metal discs 

 (as seen in fig. 322, (7), be placed between them, and let 



