If a like current is subjected to the action of a closed current _C_, the movable part will be displaced just as if it were acted upon by a force. Ampère _assumes_ that the apparent force to which this movable part _AB_ seems thus subjected, representing the action of the _C_ on the portion [alpha][beta] of the current, is the same as if [alpha][beta] were traversed by an open current, stopping at [alpha] and [beta], in place of being traversed by a closed current which after arriving at [beta] returns to [alpha] through the fixed part of the circuit. This hypothesis seems natural enough, and Ampère made it unconsciously; nevertheless _it is not necessary_, since we shall see further on that Helmholtz rejected it. However that may be, it permitted Ampère, though he had never been able to produce an open current, to enunciate the laws of the action of a closed current on an open current, or even on an element of current. The laws are simple: 1º The force which acts on an element of current is applied to this element; it is normal to the element and to the magnetic force, and proportional to the component of this magnetic force which is normal to the element. 2º The action of a closed solenoid on an element of current is null. But the electrodynamic potential has disappeared, that is to say that, when a closed current and an open current, whose intensities have been maintained constant, return to their initial positions, the total work is not null. 3. _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).