6o THE POPULAR SCIENCE MONTHLY. 



This secures the proper resetting of the apparatus in readiness for 

 a new alarm. The result is obtained very simply by making the clap- 

 per 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 j^ocket 

 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. 

 Tlie wires forming the circitit 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 Clanche battery, shown in Fig. 3, is the one universally 

 employed with this apparatus. It is very simple in construction, ex- 

 hales 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 



