Dry air is a pretty good insulator, but when the attraction between a charged and a neutral body gets great enough, the spark rips right through the air. Benjamin Franklin proved by experiment that lightning is caused by the electrification in the clouds and air. (See [Atmospheric Electricity].)
145. Theories about Electrifications. The "One-Fluid" Theory suggests that neutral bodies have a certain amount of electrification, and that they have a certain potential called zero potential. If the potential of a body becomes greater than that of the earth, the body is said to be positively electrified; if the potential of the body is less than that of the earth, it is said to be negatively electrified. If we fill a bottle with sea water, we have a great deal of water when we compare it with the bottle, but a very little water when we compare it with the sea. The earth is so large that small amounts of electrification taken from it or added to it do not affect its potential to any extent.
146. The "Two-Fluid" Theory suggests that there are two absolutely different kinds of electrification, one called positive (+), and the other negative (-). When these two are equal in quantity, the body is said to be neutral. If the body contains more + than -, the body is said to be charged positively.
It is evident then, if the two-fluid theory be accepted, that no matter how strongly a body is charged positively there must be in it some negative electrification; that is, we may charge a neutral body + by adding + electrification to it, or by taking - electrification from it. There must always be, then, some + and - electrifications in a body.
These theories do not require much consideration by the student of elementary electricity. The best thing he can do is to learn what electricity can do, and how it can be used.
EXPERIMENT 71. To study some methods of discharging an electrified body.
Apparatus. The electrophorus ([Exp. 68]); an ordinary pin ([Fig. 39]).
147. Note. You have seen sparks pass from E C to your rounded knuckle, and to other conductors. In all of these cases the discharge was sudden, one spark doing the work. Can we slowly discharge E C, or discharge it without sounds?