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- Continuous Rotations.—Among electrodynamic experiments, the most remarkable are those in which continuous rotations are produced and which are sometimes called unipolar induction experiments. A magnet may turn about its axis; a current passes first through a fixed wire, enters the magnet by the pole N, for example, passes through half the magnet, emerges by a sliding contact and reenters the fixed wire.
The magnet then begins to rotate continuously without being able ever to attain equilibrium; this is Faraday's experiment.
How is it possible? If it were a question of two circuits of invariable form, the one C fixed, the other C´ movable about an axis, this latter could never take on continuous rotation; in fact there is an electrodynamic potential; there must therefore be necessarily a position of equilibrium when this potential is a maximum.
Continuous rotations are therefore possible only when the circuit C´ is composed of two parts: one fixed, the other movable about an axis, as is the case in Faraday's experiment. Here again it is convenient to draw a distinction. The passage from the fixed to the movable part, or inversely, may take place either by simple contact (the same point of the movable part remaining constantly in contact with the same point of the fixed part), or by a sliding contact (the same point of the movable part coming successively in contact with diverse points of the fixed part).
It is only in the second case that there can be continuous rotation. This is what then happens: The system tends to take a position of equilibrium; but, when at the point of reaching that position, the sliding contact puts the movable part in communication with a new point of the fixed part; it changes the connections, it