Page:Popular Science Monthly Volume 4.djvu/604

586 to actuate, with any degree of certainty, these exposed signals, has not yet been attained. It has become necessary, therefore, to rely upon small electrical instruments, miniatures of the out-door semaphores, which direct the signal-man in the box how to exhibit his out-door signals by displaying the signals which they themselves ought to give. The same principle which produces a blow upon the bell-signal lowers the semaphore arm on the miniature to "all clear." A current of electricity, flowing through the wire of an electro-magnet, converts the iron core into a magnet, and exerts precisely the same action upon a rocking lever that a pull or strain of a signal-wire does upon the large rocking lever of a signal-post. A counter-weight, when the current ceases, restores the arm to "danger," as it does in ordinary railroad-signals. So that the miniature semaphore will remain at "all clear" so long as a current flows, but, the moment the current ceases, the arm by the action of gravity flies up to "danger." It is impossible to lower the signal at one station except by the action of an electric current, and to maintain that signal at "all clear" except by the persistent effect of the battery at the other station. The signal, therefore, is under the sole control of the signal-man toward whom the train is approaching. The instrument employed to raise and lower this miniature signal is called a "switch," from the similarity of its appearance and construction to the switch-handles or levers employed to raise and lower the larger signals on the line. Its electrical construction is precisely similar to that of the "plunger." By removing the handle over from one side to the other, it places the battery in connection with the line wire, and thereby causes a current of electricity to flow which lowers the signal.

There were four of these miniature "switches" in our signal-box, and this was the way they appeared to us to work: When the switch-handle was placed so as to be nearest us, or On, no current was transmitted, and the little signal stood at "danger;" when, however, it was pushed over farthest from us to Off, a current flowed, and the little arm was lowered to "all clear." As the arm could only be lowered when a current was flowing, it was only when the switch-handle was pushed over to Off that the "all-clear" signal could be given. Similarly, when the switch-handle was at On, the flow of electricity at once ceased, and the signal flew to "danger." The signal "all clear" could therefore only be given when the little switch was intentionally placed over to Off, and there was no other means of accomplishing this object by willfulness or accident. No accident, mechanical or electrical, could alter the miniature danger-signal. The man at our signal-box had the sole and complete control over the signal at the next box, and it was simply impossible for him to interfere with or alter the signal in his own box. This in effect is the "block" system, which answers so admirably on the principal lines of English railroad. The instructions given to signal-men who work the signals