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88 in which the potential has a sensible value is called the "electric field." A unit charge brought into any point of this field will experience a force acting in a certain direction; in the case of the field being due to the presence of a charged sphere the force acts either radially outward from or radially inward to the centre of the sphere. Where the conductor is of a more complicated shape, or where it is surrounded by a conducting surface kept at a different potential, for instance, earth potential or zero, there also corresponds to each point of the electric field a particular direction, and obviously one direction only, along which the force acts. A unit positive charge, liberated in any point of the field, will follow the impulse of that force and move from point to point along a particular line, and we may thus speak of a "line of force," meaning thereby the path along which a unit charge, or in fact any charge of positive electricity, is urged.

This conception of lines of force, as characterising the qualities of an electric field, is due to Faraday. Thus far lines of force merely have a geometrical significance, namely, that of the direction of the electric force, but it is easy to see that they must also have a