DYNAMIC GENERATION 171 it will be under the influence of the dynamic force of the current, and since force producing motion over a certain distance represents energy, it is obvious that the energy repre- sented by the displacement must have come from the current. The rate at which the energy is produced, that is the energy yielded up by the moving wire in unit time, is called " power," and since a current can only give up power if it flows in opposition to an e.m.f ., it follows that the movement of the wire across the lines of force has resulted in the generation of an e.m.f. in such a direction that it tends to diminish the strength of the current. We have here another example of Lenz's law mentioned on p. 50. To maintain the current at its old strength, we must add to the original e.m.f., which was required to overcome merely the ohmic resistance of the circuit, a further amount of e.m.f. to balance the opposing e.m.f. caused by the motion. The direction of this counter e.m.f., or generally of any e.m.f. induced by the motion of a conductor in a magnetic field, may be determined by using the right hand as an indicator. Put thumb, forefinger and middle finger again into mutual quadrature, and place the hand so that the forefinger
