Page:Electricity (1912) Kapp.djvu/70

66 Conversely, the energy thus stored can be recovered if we allow the unit charge to recede from $$A$$ to $$A_1$$, which it will do under the repelling force from the active mass $$Q$$. Positive electricity, then, tends to move from the point of higher to that of lower potential.

This is self-evident; but how does the matter stand if the sphere is charged with negative electricity? We have then not repulsion, but attraction of the unit charge. The force has changed sign. The potentials at $$A$$ and $$A_1$$ are both negative, but that at $$A$$ is more negative than that at $$A_1$$. Now by referring both to the same datum line we may also say the potential at $$A_1$$ is positive as compared to that at $$A$$. To make this matter clear, let me illustrate by substituting height for potential: On a tableland 2000 ft. above sea-level there is a mountain 500 ft. high. At the foot of the mountain a shaft is sunk 500 ft. deep. Referring vertical distances to the level of the plain we say the level of the mountain-top is +500 ft. and the level of the bottom of the mine-shaft is —500 ft.; but if we refer all heights to the sea-level we would give the mountain-top as +2500 ft. and the bottom of the shaft as +1500 ft.