Page:Popular Science Monthly Volume 89.djvu/298

 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

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��Fig. 2. Three methods of wiring which can be used with this apparatus

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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.

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