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Page:Popular Science Monthly Volume 18.djvu/68

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THE POPULAR SCIENCE MONTHLY.

given by devices on the face, which vary with different makers. In one form they are made by arrows, which lie horizontal when in normal position, and point to the names of the apartments printed above them when indicating. In another form, cards drop down in front ofPSM V18 D068 Burglar alarm area indicator.jpgFig. 1. apertures arranged in rows on the face, and in still another the name and number of a room are uncovered by a falling piece when an alarm is sounded. The needle-instrument is shown in Fig, 1, Once made, the indications remain until the parts are restored by some one, A small switch at one side completes or opens the circuit through the instrument, and one on the other side controls the connection with the bell, A row of studs at the base of the apparatus allows any opening to be disconnected that may be desired. Aside from its giving an alarm when an attempt is made to enter a building, the annunciator has an important use in showing whether a place is properly closed. If any window or door has been forgotten, it will infallibly point it out. In large business houses where there are many openings, this feature is of the greatest value. By disconnecting the bell, this test can be made a silent one.

The mechanism operating the indicators is of the simplest description. In the needle-instrument, an arm on the pivot of the needle is held in position by the hooked end of a lever, the other end of which forms the armature of an electro-magnet. The connection between the lever and the supported arm is very slight, so that a small movement of the former allows the latter to fall. When the circuit is closed this takes place. The armature in moving toward the magnet raises the hooked end of the lever, releasing the arm which drops and turns its needle. In the instrument using the card, the card is carried on the end of an arm held up in a similar manner by a hook on the armature of the magnet. The depression of the armature allows the arm to drop by its weight. The restoring of the arms to position is done by a sliding frame raised by a handle or button on the base of the instrument. Delicate as the movements of the apparatus are, it is not easily put out of order. The points of contact of the hook and arm are so made as to reduce the wear to a minimum. The mechanism is all inclosed, and the exposed parts, such as the needles, switch-handles, etc, finished in polished metal. The annunciator and bell are usually combined into one piece of apparatus, but they may be put up separate when desired.