magnetic pole is applied to that pole and is equal to the magnetic force multiplied by the magnetic mass of that pole;
(b) A very short magnetic needle tends to take the direction of the magnetic force, and the couple to which it tends to reduce is proportional to the magnetic force, the magnetic moment of the needle and the sine of the dip of the needle;
(c) If the circuit C is displaced, the work of the electrodynamic action exercised by C on C´ will be equal to the increment of the 'flow of magnetic force' which passes through the circuit.
- Action of a Closed Current on a Portion of Current.—Ampère not having been able to produce an 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, the one fixed, the other movable. The movable part was, for instance, a movable wire αβ whose extremities α and β could slide along a fixed wire. In one of the positions of the movable wire, the end α rested on the A of the fixed wire and the extremity β on the point B of the fixed wire. The current circulated from α to β, that is to say, from A to B along the movable wire, and then it returned from B to A along the fixed wire. This current was therefore closed.
In a second position, the movable wire having slipped, the extremity α rested on another point A´ of the fixed wire, and the extremity β on another point B´ of the fixed wire. The current circulated then from α to β, that is to