Insulation and earth-connections \ 3 5 



electrified, though even in no greater degree than in these experiments, 

 and kept so for a second or two, and its electricity then destroyed, the 

 air near it will be sensibly overcharged, as may be thus shewn. Take a 

 pair of pith balls, like those hung at D, and suspend them within a few 

 feet of the wire from some body communicating with the ground. The 

 balls will instantly separate on electrifying the wire on account of the 

 repulsion of the redundant fluid in it, but they will also continue to 

 separate, though in a less degree, after the electricity of the wire is de- 

 stroyed, which can be owing only to the air being rendered overcharged 

 by it. 



257] It may be suspected that this electrification of the air by the 

 wires may affect the separation of the pith balls at D and thereby cause 

 an irregularity in the experiments, but it must be considered that the 

 wire mMNn is made as much undercharged as rRSs is overcharged, and 

 the pith balls are placed about equally distant from both, so that the 

 undercharged air near one wire will nearly balance the effect of the over- 

 charged air near the other. Besides that, if it had any effect upon the 

 separation of the balls, it would have much the same effect in trying B 

 as in trying b, and therefore could hardly cause any error in the result of 

 the experiment. However, still further to obviate any error from that 

 cause, I had a contrivance by which the electricity of the wires rRSs and 

 mMNn, as well as that of the vials, was destroyed as soon as the wires 

 rR and mM were lifted up from B and T. 



258] It is necessary that the outside of the bottle A and the wire yx 

 should have as perfect a communication with the ground as possible, as 

 otherwise it might happen that the body B and the trial plate might not 

 receive their full degree of electrification before the wires rR and mM 

 were lifted up. I therefore made them to communicate by a piece of wire 

 with the outside wall of the house. This I found to be sufficient, for if 

 I charged a vial, making the outside to communicate with the outside 

 wall, and then made a communication by another wire between the inside 

 of the vial and another portion of the outside wall of the house at several 

 feet distance from the other, I found the vial to be discharged instantly; 

 but if I made the wires to communicate only with the floor of the room 

 instead of the wall of the house, I found it took up some time before the 

 vial was discharged. 



It must be observed that in this case, where you want to carry off 

 the electricity very fast by an imperfect conductor, such as the wall, the 

 best way is to apply a pretty broad piece of metal to the wall, so as to 

 touch it in a considerable surface, and to fasten the wire to that, which 

 was the way I last made use of, for if you only apply the wire against the 

 wall, as it will touch the wall only in a few points, the electricity will not 

 escape near so fast. 



