[Illustration:  FIG. 1.]

In order to prevent oscillations, which could not fail to occur after each emission of a current (so that the helix, instead of returning to a position of equilibrium and stopping there, would go beyond it and alternately uncover the slits, a a’), the apparatus is provided with a liquid deadener.  To this end, the prolongation, v, carries a piece, o, which dips into a cup containing a mixture of glycerine and water.

We shall now describe the differenzialrecorder.  Opposite the two slits, a and a’, there are two powerful converging lenses, l and l’, whose foci coincide with two sorts of selenium plate rheostat, z and z’.  The result of this arrangement is that as soon as one of the slits, as a consequence of the displacement of the helix, r, allows a luminous fascicle to escape, this latter falls upon the corresponding lens, which concentrates it and sends it to the selenium plates just mentioned.  Under the influence of the luminous rays, the resistance that the selenium offers to the passage of an electric current instantly changes.  At M and M’ are placed two horseshoe magnets whose poles are provided with pieces of soft iron that serve as cores to exceedingly fine wire bobbins, d.  These polarized pieces are arranged in the shape of a St. Andrew’s cross, and in such a way that the poles of the same name occupy the two extremities of the same arm of the cross, an arrangement very clearly shown in Fig. 2.

[Illustration:  FIG. 2.]

Between the poles of the magnets, M and M’, there is a permanent magnet, A, movable around a vertical axis, i.  Four spiral springs, f, whose tension may be regulated, permit of centering this latter piece in such a way that when the current is traversing the spirals of the polar bobbins it is equally distant from the four poles, n, s, s’, and n’.  Under such circumstances it is evident that a difference in the power of attraction of these four poles, however feeble it be, will result in moving the magnet, A, in one direction or the other around its axis.  The energy and extent of such motion may, moreover, be magnified by properly acting upon the four regulating springs.

The bobbins of the magnet, M, are mounted in series with the selenium plates, z, the local battery, B, and a resistance box, W. Those of the magnet, M’, are in series with z’, B’, and W’.  The local batteries, B and B’, are composed of quite a large number of elements.  The current from the battery, B, traverses the selenium plates and the bobbins of the magnet, M, and returns to B through the rheostat, W; and the same occurs with the current from B’.  The two currents, then, are absolutely independent of one another.