ELECTRICAL CONDENSERS. 613 



the upper disc be charged by drawing a rubbed glass 

 rod along the wire fixed upon it. The charge received 

 will not be sensibly greater than it was before intro- 

 ducing the insulating disc. Very different results will, 

 however, be obtained, if 



First, the lower disc is connected with the table and 

 thus with the ground by means of a wire about 15 cm 

 long, one end of which is bent into a hook which is 

 attached to the wire fixed in the lower disc. When 

 electricity is now communicated to the upper disc$ the 

 pendulum rises but slowly $ and the rubbed glass rod 

 must be drawn repeatedly along the wire on the disc, 

 that the divergence of the pendulum may be the same 

 as it was in the first experiments. The upper disc 

 having been charged in this manner, let the thumb be 

 placed on the lower disc and the forefinger be brought 

 near the upper disc: a spark will pass between the 

 upper disc and the finger; it is not longer than the 

 sparks drawn in the first experiments, but is brighter^ 

 louder, and can be much more distinctly felt* While the 

 previous sparks were hardly perceptible to the feeling, 

 a smart shock is now felt in the joints of the fingers. 



Second, the apparatus being charged exactly as in 

 the preceding experiment, the wire which connects the 

 lower disc with the table is removed, the upper disc k 

 lifted by its handle of sealing-wax, and the insulating 

 disc is also taken down, care being taken to touch 

 neither of the metallic discs with the finger. On test- 

 ing the lower disc by means of ar charged electroscope it 

 will be found to be negatively electrified, if, as has been 

 supposed in the experiment, the upper disc was charged 

 positively. 



