58 



THE POPULAR SCIENCE MONTHLY. 



Fig. 1. 



given by devices on the face, which vary with different makers. In 

 one form they are made by arrows, which lie horizontal Avhen in nor- 

 mal position, and point to the names of the apartments j^rinted above 

 them when indicating. In another form, cards drop down in front of 



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 con- 

 nection 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 build- 

 ing, the annunciator has an important 

 use in showing whether a place is proper- 

 ly closed. If any window or door has 

 been forgotten, it will infallibly point it 

 out. In large business houses where there are many oj^enings, this 

 feature is of the greatest value. By disconnecting the bell, this test 

 can be made a silent one. 



The mechanism ojierating the indicators is of the simplest descrip- 

 tion. In the needle-instrument, an arm on the j^ivot of the needle is 

 held in jjosition 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 move- 

 ment 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 appara- 

 tus 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 annunci- 

 ator and bell are usually combined into one piece of apparatus, but 

 they may be put up separate when desired. 



