The mechanism that controls the motions of the upper rod that supports the carbon-holder consists of two bobbins of fine wire, E (Fig. 2), mounted on a derived circuit on the terminals of the lamp; of a lever, L, articulated at O, and supporting a tube, TT', and the whole movable part balanced by a counterpoise, P. This lever, P, carries two soft iron cores, F, which enter the bobbins, E, and become magnetized under the influence of the current that passes through them. The upper part of the tube, T, carries a square upon which is articulated at O' a second lever, L', balanced by a second counterpoise, P', and carrying a flat armature, _p_, opposite the cores, F', that are fixed to the first horizontal lever, L. The carbon-holder rod, CC', slides freely in the tube, TT', and is wedged therein by a small piece, _a m l_, fixed to the lever, L'. For this reason the tube, TT', is provided with a notch opposite the piece _a m l_, and the two arms, _a_ and _m_, of the latter are shaped like a V, as may be seen in part in the plan in Fig. 2. It is now easy to understand how the system operates; when the current is not traversing the circuit, the carbons are separated; but, at the moment the circuit is closed for lighting a series of lamps, it traverses the electro-magnet, which then becomes very powerful, and draws down the cores, F, along with the lever, L, the tube, TT', and the carbon-holder, CC', and brings the carbons in contact. The arc then forms, and the current divides between the arc and the bobbins, E. Its action upon the cores, F, becomes weak, and it can no longer balance the counterpoise, P, which falls back, and raises the system again. The arc thus becomes _primed_. The cores, F, however, preserve a certain amount of magnetization; the armature, _p_, is attracted, and the lever, L', assumes a position of equilibrium such that the piece, _a m l_, wedges the rod, CC', in the tube, TT', and holds it suspended. When, through wear of the carbons, the arc elongates, a greater portion of the current passes into the bobbins, E, the armature, _p_, is attracted with more force, and the lever, L', swings around the point, O'. The rotation of L' separates the piece, _a m l_, from the rod, CC', which, being thus set free, slides by its own weight and shortens the arc. The current then becomes weak in E, the armature, _p_, is not so strongly attracted, the lever, L', pivots slightly around O' under the action of the weight, P', and the brake or wedge enters the notch anew, and stops the descent of the carbon. In practice, the motions that we have just described are exceedingly slight; the carbon moves imperceptibly, and the length of the arc remains invariable. [Illustration: Fig. 1--MONDOS'S ELECTRIC LAMP.] It will be seen, then, that the lever, L, and the tube, TT', serve exclusively for _lighting_, and the lever, L', exclusively for regulating the distance of the carbons.