62 THE POPULAR SCIENCE MONTHLY. 



tons 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 i^articular case of attempted 

 robbery, the general immunity from such attempts that the presence 

 of the apparatus secures is of still gi-eater 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 bat- 

 tery 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 burg- 

 lars, accustomed to take large chances, might frequently overpower 

 the watchman and stop the alarm before it excited outside atten- 

 tion. To meet this difficulty the j^lan is sometimes adopted of mak- 

 ing 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 con- 

 nected on a closed circuit with the central office, at which place deli- 

 cate galvanometers are used as indicators. The circuit of each build- 

 ing 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 cir- 

 cuit-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 

 wii'es 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 prac- 

 ticed to some extent. The apparatus used in this case is much less 

 delicate, and the protection therefore not so good. 



