The starting apparatus consists of a small galvanometric helix, _r_, analogous to Thomson's siphon recorder, which is suspended from a cocoon fiber and capable of moving in an extremely powerful magnetic field, N S. This helix carries, as may be seen in Figs. 1, 3 and 4, a prolongation, _v_, at its lower end whose form is that of a prism, and which is arranged in front of the partition of the box, K, in such a way that it exactly covers the two slits, a and _a_ when the bobbin is at rest, and in this case prevents the luminous rays of the lamp, _g_, from escaping from the box. But, as soon as the current sent through the cable reaches the spirals of the bobbin, through the conductors, _y y'_, the sum of the elementary electrodynamic actions that arise causes the helix to revolve to the right or left, according to the polarity of the current, while at the same time the helix slightly approaches one or the other of the poles of the magnet. The prolongation, _v_, of the helix, being firmly united with the latter, follows it in its motion, and has the effect of permitting the luminous rays to escape through one or the other of the slits, _a a'_, so that the freeing of the luminous fascicle, if such an expression is allowable, is effected. [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.]