Page:Electricity (1912) Kapp.djvu/65

Rh This force diminishes rapidly as we increase the distance; at 10 times the distance it is $$\frac{1}{100}$$ at 100 times the distance it is $$\frac{1}{10000}$$ the original value. Obviously if the room is large enough it may be practically zero close to the wall, and yet quite sensible within a foot or so of the large sphere.

Let us assume that somehow or other we pick up a pith ball charged with unit positive electricity and carry it along any path to some point near the sphere. All the way we experience a repelling force, small at first, but rapidly increasing as we approach to the final position. In overcoming this repelling force we must impress mechanical energy on the pith ball, and the energy thus stored can again be recovered in letting the pith ball recede and perform mechanical work by overcoming some opposing force, so regulated that it balances at any point the repelling force of electricity.

We need not concern ourselves with the mechanism by which such a process could be carried out, since the whole experiment is only hypothetical and merely intended to illustrate principles. Our unit charge then is a carrier of energy, or a means of storing energy; and the amount of energy stored will depend on the charge on the sphere,