Page:Popular Science Monthly Volume 18.djvu/72

62 under the carpet or circuit-closers in interior doors will reveal the burglar's presence in perhaps every case.

Valuable as is the protection in any particular case of attempted robbery, the general immunity from such attempts that the presence of the apparatus secures is of still greater moment. Burglars will not generally take such risks as those imposed by an efficient alarm system, and will therefore give a house so protected a wide berth. The only case in which there is room for failure of the system is when the battery power is not sufficient to operate the alarm. But it is a very simple matter to provide against this. Tests once every month or two, and the experience soon gained in using the battery, will enable one to know at any time the state of the system. None of the other parts need ever cause any solicitude.

While in the great majority of cases the plan of giving the alarm to some one in the building broken into affords perfect security, in some it does not. In business centers, determined and cunning burglars, accustomed to take large chances, might frequently overpower the watchman and stop the alarm before it excited outside attention. To meet this difficulty the plan is sometimes adopted of making the alarm sound in a central office of the company furnishing the apparatus. One company doing this has adopted a system that seems to be beyond circumvention. Each building protected is connected on a closed circuit with the central office, at which place delicate galvanometers are used as indicators. The circuit of each building is independent of all others. Any change in the resistance of any circuit is instantly shown by the deflection of the proper needle, and an alarm started. The opening of a protected door or window breaks the circuit, as does the cutting of the line, and of course gives an alarm. If the burglar could carry the wire to the ground and insert just the proper resistance, no signal would be given at the company's office, but this is impossible, as the resistance is not only that of the wire but of the apparatus in circuit. The only way to get around it is to tunnel under the building, but even then circuit-breakers judiciously disposed would generally lead to detection. Nothing is gained, so far as the safe is concerned, in this case, as it is independently protected. It is placed in a light wooden cabinet lined with a metallic casing, consisting of two sheets of tin-foil insulated from each other by a thin sheet of non-conducting material. The wires from a battery are connected each with one of the sheets of foil. So delicate is the insulation that the sticking of a pin in the cabinet closes the circuit and deflects the needle, and sounds the alarm in the central office. This system, though not yet in extensive use, is gaining in favor among merchants having valuable stores of goods. A similar plan of protecting private houses whose occupants are away is practiced to some extent. The apparatus used in this case is much less delicate, and the protection therefore not so good.