When one of the buttons, b, is pressed, one of the levers of the key-board arrangement touches the disk, M, which is insulated from the other portions of the key-board, and the current then passes from the terminal C to M, and there bifurcating, one portion of it goes to the bobbins of the apparatus and thence to the earth, while the other goes to actuate the correspondence apparatus.  The index-hands of the two apparatus thereupon begin their movement simultaneously, and only stop when the pressure is removed from the button and the current is consequently interrupted.  H is a ratchet-wheel, which, like the key-board, is insulated from the rest of the apparatus.  The button, K, located over each of the dials, serves to bring the index-needles back to their position under the cross shown in Fig. 12.  The key, X, serves for winding up the clock-work movement.

The Annunciator Apparatus.—­This apparatus, which performs the same role as the one just described, is simply an ingenious modification of the annunciator used in hotels, etc.

It consists of a wooden case, containing as many buttons as there are phrases to be exchanged.  Over each button, b, there is a circular aperture, behind which drops the disk containing the phrase.  Between the buttons and the apertures are rectangular plates, P, in which are inscribed the answers given by pressing on the button of the receiving tablet—­a pressure which, at the same time, removes the corresponding disk from the aperture.  Two disks located at the upper part carry these inscriptions:  “Error, I repeat;” “Wait.”  The tablets on exhibition have eight disks, and can thus be used for exchanging six different phrases.  In the interior, opposite each aperture, there is a Hughes magnet, between the arms of which there oscillates a vertical soft-iron rod, carrying a disk.  The maneuver “is simple.”  By pressing upon a button there is sent into the bobbins of the magnet corresponding to this button a current which causes the disk to appear before one of the apertures, while at the same time an alarm begins to ring.  The same maneuver performed by the agent at the receiving-post has the effect of causing the disk to disappear.  The two contact springs in communication at each aperture with the alarm and the line are connected by a strip of ebonite, M, against the center of which presses the button.

Electrical Controllers for Water-Tanks.—­The object of these apparatus is to warn the person in charge of a water-tank that the latter is full, and that he must stop the engine-pump; or, that the tank is empty, and that he must at once proceed to fill it.  The Company of the North has on exhibition two such apparatus—­one of them Lartigue’s, and the other Verite’s.

1. The Lartigue Controller (Fig. 15).—­This apparatus consists of a long lever, A, which carries at one of its extremities a funnel, E, having a very narrow orifice and which is placed under the overflow pipe of the tank.  The lever is kept normally in a horizontal position by a counterpoise; but, as soon as the overflow runs into the funnel, the weight of the water tilts the lever, and the mercurial commutator, F, closes the circuit of a pile, which actuates an alarm-bell located near the pump and engine.  The two stops, a and a’, limit the play of the lever.