This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
We herewith illustrate an exceedingly simple form of detecter, to show if the night watchmen perform their visits regularly and punctually. In the case, C, is a clockwork apparatus driving the axle, S, at the end of which is a worm which gears into the wheel of the drum, D. The rotation of D, thus obtained unrolls a strip of paper from the other drum, D. This paper passes over the poles of as many electro-magnets as there are points to be visited, and underneath the armatures of these electro-magnets. Each armature has a sharp point fixed on its under side, and when a current passing through the coils causes the attraction of the armature, this point perforates the paper. The places to be visited are connected electrically with the binding screws shown, and the watchman has merely to press a button to make the electric circuit complete. It has been found in practice that plain paper answers every purpose, as the clock giving an almost uniform motion enables the reader, after having seen the perforated slips once or twice, to determine fairly well the time which elapses between each pressure of the button. - The Engineer.