454
EXPOSITION AT VIENNA.
of the magnet, and their attraction counterbalances the action of the spring.
If a current of electricity is made to pass through the helices in a certain direction the armature is momentarily repelled, the lever falls, and the whistle sounds until the engineer, by pressing on a button which is shown on the under side of the box (Plate D), arrests it in returning the lever to its original position (i. e., in contact w T ith the magnet).
The current of electricity is produced in the following manner : —
The wire of the magnet is connected on one side with the body of the engine and by the intermediary of the wheels and rails with the ground. The other extremity is prolonged by a wire which, descending under the engine, is connected to a metallic brush , insulated and fixed in such a position that the end is lower by several centimetres than any projection on the engine.
This brush (Figs. A and B) is composed of stiff brass wires, of about No. 8 gauge, strongly set in an insulating substance, but terminating at their upper ends in a brass plate, which .is again protected on the upper side by insulation. The connection is made by wire to this plate.
On the track, at any desired distance from the disc or semaphore, is placed a "fixed contact,” composed of a piece of w r ood (see Fig. C), placed longitudinally between the rails and supported by iron standards at such a height as not to be touched by any projection on the engine.
This piece of wood, covered with an insulating compound, has on its upper surface a sheet of brass, which, by means of a wire of any desired length, is placed in communication wfith the positive pole of a galvanic battery (see Plate B). The negative pole is connected to a commutator fixed on the semaphore in a manner similar to that used in Mayer & Wolff’s system (see Plate 4), which connects it with the ground when the disc is turned to " arrest.” (The " son- nerie ” shown in Fig. B is the station-alarm, and has no connection with this apparatus.)
On the passage of the engine the brush presses strongly against the fixed contact. If the distant semaphore is at