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Page:Popular Science Monthly Volume 18.djvu/73

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THE ELECTRIC BURGLAR-ALARM.

The cost of applying the burglar-alarm to any house will vary in each case. It depends upon the size of the annunciator required and the number of openings to be protected. The prices charged by the different American manufacturers differ very little. Annunciators range in price from thirty dollars with four indications to one hundred dollars with twenty. The annunciator used should have as many indications as there are rooms protected. The cost of circuit-closers, including the placing in position and laying the wire, is three dollars a window when both sashes are connected. The same devices for doors vary from one and a half to two and a half dollars. In ordinary city houses it is only necessary to connect the windows and doors, front and back, of the first two stories and the opening in the roof. The entire cost will not generally exceed one hundred dollars. In the country the cost would of course be somewhat greater, in the average house probably between a hundred and fifty and two hundred dollars. The apparatus once in, the only expense is the maintenance of the battery. This will generally be very small, probably not more than a dollar a year. Considering the security gained, the outlay required is not excessive, and builders find that it is fully made up to them in increased rents. It is not improbable that the apparatus will eventually be considered as necessary to the complete equipment of a house as now are water-and gas-pipes.

Intimately connected with the burglar-alarm system, though having a different object, is an automatic fire-alarm, somewhat extensively introduced during recent years. The system consists in placing in the ceiling of a room a number of mercury bulbs, which close an electric circuit when the temperature rises above a certain point and set off an alarm. One form of the bulb or thermostat is shown in Fig. 4. The PSM V18 D073 Fire alarm thermostat.jpgFig.—4. Thermostat. wire from the lower end is in permanent contact with the mercury, but that in the upper end comes in contact with it only when a given temperature is reached. The bulbs are usually manufactured to make this contact at a temperature of 120° Fahr. The thermostat is set in a bell-shaped shield of sheet-metal, only the rim of which, when in the ceiling, is exposed. They are placed about twenty feet apart in large rooms, a couple being sufficient for those of ordinary size. This alarm, like that for burglars, may be complete in a building, or may give its signal at a central office. In some of the larger cities this latter plan has been carried into practice. Each building is provided with an annunciator placed on the front, where it can be readily inspected. The signal given at the office indicates the building, and the annunciator on the face of the building gives the room in which the fire is located.