284 



Popular Science Monthly 



Making an Electric Fire Alarm 



ASVSTKM of fire warning illustrated 

 herewith may be installed at little 

 expense. The installation makes use of 

 mercury, and it can be quickly adjusted 

 and requires no resetting. The detecting 

 mechanism is clearly shown in Fig. i. 

 It consists first of a glass tube A about 

 2 ins. long and having a bore of i/8 in. 

 or a triHe more. A short piece 

 of brass rod that just fits the 

 tube has a piece of copper wire 

 soldered to it 4 ins. long. The 

 rod is fitted into the tube as 

 shown at B and the end of the 

 tube is filled with sealing wax 

 C. The rod should not fit too 

 tight or its expansion will 

 break the tube. 



A small rubber cork that fits 

 the tube tightly has a needle 

 run through it. A short length 

 of wire should be soldered to 

 this needle as shown in the 

 drawing. The tube is fastened 

 to a base measuring I >^ ins. by 3 

 ins. by means of a brass strip F 

 and two small screws. Binding posts 

 are mounted on the base and the wires 

 connected to same. 



A few drops of mercury are put in the 

 tube and the rubber cork and needle 

 inserted as shown at D and E. By 

 regulating the distance between the 

 needle and the surface of the mercury 

 the temperature at which the alarm is 

 given can be regulated. 



Fig. 1. The fire 

 detecting mech- 

 anism complete 



A good method of adjusting this 

 apparatus is to connect a bell and 

 battery to the terminals. Place the 

 instrument and a thermometer in an 

 oven or sand bath and adjust the 

 needle so the bell just rings when the 

 thermometer registers 110 degrees F. 



In Fig. 2 is given three wiring diagrams 

 that may be used with this apparatus. 

 At A is shown the fire detectors 

 connected in with the regular 

 burglar alarm system which is 

 the easiest way of installing the 

 apparatus. At B a third wire 

 is run and two bells are used so 

 that the householder can de- 

 termine whether it is a fire or 

 burglar alarm. The bells should 

 give a different note and this 

 can be done by placing a wad 

 of paper under one or by sawing 

 a slot. The third diagram C 

 shows the instruments con- 

 nected for fire alarm onh-. 



An added feature refinement 

 would be an indicator to show 

 at a glance where the alarm is 

 from. Note particularly the 



^||li^— 

 _^<. 



Fig. 2. Three methods of wiring which 

 can be used with this apparatus 



commg 



three-point switch in diagram C which 

 enables the owner to test the bell and 

 batteries by placing it on point i. Pla- 

 cing it on point 2 puts the system into 

 service and point 3 can be used as a 

 ground test for the wiring. 



A New Detector Material 



SIXCE the mineral t\pe of detector 

 was first discovered, attempts have 

 been made to use artificial substances in 

 place of the natural minerals, but as a 

 rule these preparations are much less 

 .sensitive. A European inventor claims 

 to have found a mixture that produces 

 a \-ery good substitute for mineral, and 

 which is quite as sensitive. Besides, the 

 effect is claimed to be uniform all o\-er 

 the surface. To make this, mix two 

 parts of puhcrized galena with one [)art 

 of natural mineral siK'er in the shape of 

 filings, then add stibine and sulphide of 

 siKer. When an intimate mixture has 

 been obtained, put into a test tube and 

 heat to white lieat. Temper while hot 

 in ammonia water and remove the 

 compact mass, which may then be 

 mounted for the detector in the same 

 way as a piece of mimrai. 



