of magnetic force" which passes through the circuit.
2. Action of a Closed Current on a Portion of Current.—Ampère being unable to produce the open current properly so called, had only one way of studying the action of a closed current on a portion of current. This was by operating on a circuit C composed of two parts, one movable and the other fixed. The movable part was, for instance, a movable wire αβ, the ends α and β of which could slide along a fixed wire. In one of the positions of the movable wire the end α rested on the point A, and the end β on the point B of the fixed wire. The current ran from α to β—i.e., from A to B along the movable wire, and then from B to A along the fixed wire. This current was therefore closed.
In the second position, the movable wire having slipped, the points α and β were respectively at A' and B' on the fixed wire. The current ran from α to β—i.e., from A′ to B′ on the movable wire, and returned from B′ to B, and then from B to A, and then from A to A′—all on the fixed wire. This current was also closed. If a similar circuit be exposed to the action of a closed current C, the movable part will be displaced just as if it were acted on by a force. Ampère admits that the force, apparently acting on the movable part AB, representing the action of C on the portion αβ of the current, remains the same whether an open current runs through αβ,
