Page:Popular Science Monthly Volume 18.djvu/70

60 This secures the proper resetting of the apparatus in readiness for a new alarm. The result is obtained very simply by making the clapper turn a switch, which cuts from the circuit the opened window or door, and allows the current to pass directly from the battery to the bell.

The door and window attachments for closing the circuit by the movements of these parts are of various forms. Those used on doors are simply little push-pins placed in the casing. The pin slides in an insulated case provided with metallic strips. When it is pressed in, the contact between it and the strips is broken and the circuit opened. When the pressure is released, the pin springs out, closing the circuit. The slightest movement of a door allows this motion of the pin to take place. In one form the pin and a metallic casing are so arranged that the attempt to keep the pin pushed in, when the door is opened, by inserting a knife-blade, establishes the circuit and gives the alarm. These push-buttons may be constructed to close the circuit, either by pushing in or springing out, and in both forms have a great variety of uses. They may be placed under the carpet, in the hall, on the stairs, in front of a window, or wherever any one entering would be liable to step. A sufficient number properly disposed could make intrusion without giving an alarm simply impossible. The window attachments are usually simple springs placed in the casing so that the movement of the sash presses them together. One form consists of a roller on the the end of a spring arm, which keeps it pressed out from contact with a metal strip, through which the circuit is completed. Placed in the casing, the roller stands out and is received in a pocket in the edge of the sash, so that the motion of the sash brings the roller arm and metal strip into contact. For the purpose of ventilation, the pocket in the upper sash is usually elongated to give a free movement through any desired distance. When the lower sash is left open, security can be gained by covering a push-pin in the window-sill with a flower-pot or other obstruction, the removal of which is necessary to gain entrance. The wires forming the circuit are of insulated copper, carefully put up so as to be completely hidden from view. They are run in grooves in the wood-work, carried beneath a floor, or on its face, according to circumstances. Once in place, they remain unchanged for any period, causing neither trouble nor expense.

The Le Clanché battery, shown in Fig. 3, is the one universally employed with this apparatus. It is very simple in construction, exhales no noxious gases when in operation, does not waste the material when no current is passing, and needs but very little attention. The positive pole is a piece of gas carbon placed in a porous cell filled with coarse-grained peroxide of manganese and carbon. The cell is sealed at the top with pitch, and a lead cap on the carbon receives the wire. The negative pole is formed of a rod of amalgamated zinc. Both poles are immersed in a solution of sal-ammoniac contained in a glass