If we suppose (Fig. 1) a magnetized needle, _n s_, placed between the extremities of a soft iron core, N S, and if we group the circuit in such a way that the current, after traversing the coil, _e e_, of the electro, traverses a circle, _d d_, situated in a plane at right angles with the plane of the needle's oscillation, it is evident that we shall have obtained an apparatus that satisfies the aforesaid conditions. It seems at first sight that in such an instrument the directing force should be constant from the moment the electro was saturated, and it would be possible, were sufficiently thin cores used, to obtain a constancy in the directing magnetic field for relatively feeble intensities. In reality, the actions are more complex. The needle, _n s_, is, in fact, induced to return to its position of equilibrium by two forces, the first of which (the attraction of the poles, N S) rapidly increases with the intensity so as to become quickly and perceptibly constant, while the second (the sum of the elementary electrodynamic actions that are exerted between the spirals, _e e_, and the needle, _n s_) increases proportionally to the intensity of the current. If we represent these two sections graphically by referring the magnetic moments as ordinates and the current intensities as abscissas to two co-ordinate axes (Fig. 2), we shall obtain for the first force the curve, O A B, which, starting from A, becomes sensibly parallel with the axis of X, and for the second the right line, O D. The resultant action is represented by the curve, O E E'_F. It will be seen that this action, far from being constant, increases quite rapidly with the intensity of the current, so that the deflections would become feebler and feebler for strong intensities, of current; and this, as well known, would render the apparatus very defective from a practical point of view. [Illustration: FIG. 2.] But the action of the spirals can be annulled without sensibly diminishing the magnetism of the core by arranging a second system of spirals identical with the first, but placed in a plane at right angles therewith, or, more simply still, by having a single system of spirals comprising the coil of the electro-magnet, but distributed in a plane that is oblique with respect to the needle's position of rest. It then becomes possible, by properly modifying such angle of inclination, to obtain a total directing action that shall continue to increase with the intensity, and which, graphically represented, shall give the curve, O G G'_H, for example (Fig. 2). [Illustration: FIG. 3.] [Illustration: FIG. 4.] [Illustration: FIG. 5.]