Page:Elementary Text-book of Physics (Anthony, 1897).djvu/362

348 potential of the pole is positive when the current, as seen from the pole, is directed counterclockwise, that is, when the face of the circuit which confronts the pole is its positive face, which corresponds to the north face of a magnetic shell. The energy of the circuit is also positive in this position. It becomes zero when the pole is brought into the plane of the circuit outside of it, and negative when the pole confronts the negative face. The energy of the circuit is therefore diminished by turning its negative face toward the pole and moving it up toward the pole. The tubes of induction of the pole then pass through the circuit in the positive direction, that is, in the same direction as the tubes of the circuit; and the motion is such as to include as many of the tubes of the pole in the circuit as possible. The rule thus illustrated is a general one: a circuit in a magnetic field tends to move so that as many of the tubes of the field as possible pass through it in the positive direction; or, more fully, it tends to move so that the difference between the tubes which pass through it in the positive direction and in the negative direction is as great as possible. In terms of the symbols already used, the motion is such as to make $$N$$ negative and numerically as great as possible. From this rule it is easy to see that a circuit will be in stable equilibrium with a soft iron bar when the axis of the bar is normal to the circuit, the tubes of induction in the bar are in the same direction as those of the circuit, and the bar is as near the edge of the circuit as possible.

When the field is due to the presence of another circuit the motion is such as to set their tubes of induction in the same direction, and to include in each circuit as many of the tubes of the other as possible; that is, to make $$M$$ negative and numerically as great as possible. When the circuits are thus placed, their currents are travelling in the same sense. Their mutual action may therefore be expressed by saying that currents travelling in the same sense attract, and in opposite senses repel, each other.

The action on a circuit in a field due to magnets, or the mutual action of two circuits, may be described in terms of the actions that would be exerted on the magnetic shells which are equivalent to