Page:Electricity (1912) Kapp.djvu/66

the medium in which the approach takes place, and the distance from the centre of the sphere at which the approach is arrested. Thus to every point of space surrounding the sphere corresponds a definite amount of energy. The nearer the point is to the surface of the sphere, the greater is the amount of energy required to bring unit charge to that point. By moving the pith ball nearer to the sphere we must expend energy, that is, store it; by allowing it to move farther away we obtain energy, that is, we diminish the amount stored. If we move the pith ball round the sphere, taking care to keep at the same distance, we neither expend nor receive energy. In this case the movement takes place everywhere at right angles to the direction of the force, and consequently no work can be done. Our pith ball is only potent to give up energy if allowed to recede from the sphere in obedience to its repelling force, and the measure of this "potency," or, as we may shortly term it, the "potential," is a measure of the total energy which the pith ball yields if allowed to move from the point in question to a point so far away that the force has dwindled to zero—in mathematical language, to a point infinitely distant. The