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 changes therefore the conditions of equilibrium, so that the position of equilibrium fleeing, so to say, before the system which seeks to attain it, rotation may take place indefinitely. Ampère assumes that the action of the circuit on the movable part of _C'_ is the same as if the fixed part of _C'_ did not exist, and therefore as if the current passing through the movable part were open. He concludes therefore that the action of a closed on an open current, or inversely that of an open current on a closed current, may give rise to a continuous rotation. But this conclusion depends on the hypothesis I have enunciated and which, as I said above, is not admitted by Helmholtz. 4. _Mutual Action of Two Open Currents._--In what concerns the mutual actions of two open currents, and in particular that of two elements of current, all experiment breaks down. Ampère has recourse to hypothesis. He supposes: 1º That the mutual action of two elements reduces to a force acting along their join; 2º That the action of two closed currents is the resultant of the mutual actions of their diverse elements, which are besides the same as if these elements were isolated. What is remarkable is that here again Ampère makes these hypotheses unconsciously. However that may be, these two hypotheses, together with the experiments on closed currents, suffice to determine completely the law of the mutual action of two elements. But then most of the simple laws we have met in the case of closed currents are no longer true. In the first place, there is no electrodynamic potential; nor was there any, as we have seen, in the case of a closed current acting on an open current. Next there is, properly speaking, no magnetic force. And, in fact, we have given above three different definitions of this force: 1º By the action on a magnetic pole; 2º By the director couple which orientates the magnetic needle; 3º By the action on an element of current. But in the case which now occupies us, not only these three definitions are no longer in harmony, but each has lost its meaning, and in fact: 1º A magnetic pole is no longer acted upon simply by a single force applied to this pole. We have seen in fact that the force due to the action of an element of current on a pole is not applied to the pole, but to the element; it may moreover be replaced by a force applied to the pole and by a couple;