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 stopping at α and β, or whether a closed current runs first to β, and then returns to α through the fixed portion of the circuit. This hypothesis seemed natural enough, and Ampère innocently assumed it; nevertheless the hypothesis is not a necessity, for we shall presently see that Helmholtz rejected it. However that may be, it enabled Ampère, although he had never produced an open current, to lay down the laws of the action of a closed current on an open current, or even on an element of current. They are simple:

(1) The force acting on an element of current is applied to that element; it is normal to the element and to the magnetic force, and proportional to that component of the magnetic force which is normal to the element.

(2) The action of a closed solenoid on an element of current is zero. But the electro-dynamic potential has disappeared—i.e., when a closed and an open current of constant intensities return to their initial positions, the total work done is not zero.

3. Continuous Rotations.—The most remarkable electro-dynamical experiments are those in which continuous rotations are produced, and which are called unipolar induction experiments. A magnet may turn about its axis; a current passes first through a fixed wire and then enters the magnet by the pole N, for instance, passes through half the magnet, and emerges by a sliding contact and re-enters the fixed wire. The magnet