dynamic significance. Obviously if we move along a line of force, the actual magnitude of the mechanical force experienced by the unit charge does not necessarily remain the same. Take the simple case of the field due to a charged sphere hanging free in space. The lines of force are all straight lines converging to the centre of the sphere. If at a distance of one yard our unit charge is repelled with a certain force, then at a distance of half a yard the force would be quadrupled. Thus, although we may travel along one and the same line of force, the magnitude of the force changes inversely as the square of the distance. As we approach the sphere, the potential increases inversely as the first power (not the square) of the distance. Potential and force are two things of different character, namely, energy and mechanical force respectively. We have seen that potential may be considered as an attribute of space, and the idea lies near to also consider electric force as an attribute of space, although as an attribute of a different character. This attribute is a mechanical force, namely, the force exerted on unit positive charge.
Let us see how we could make a mechanical model of the lines of force emanating from a
