How Bi-Metallic Strips & a Bell Can Be Used to Make a Fire Alarm

Written by k.k. lowell
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  • Introduction

    How Bi-Metallic Strips & a Bell Can Be Used to Make a Fire Alarm

    A bi-metallic strip is a metal strip made of two metals which have different expansion rates. These metals are fused solidly together. When heated, the strip curls due to the greater expansion rate of one of the two fused metals. This property has long been used in thermostats and thermal switches. This property also makes it possible to make a very simple fire alarm using a bi-metallic strip, a battery and a bell.

    Base Layout ()

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    Use a small piece of wood as a base for your alarm. (The dimensions of the base will be dependent on the size of the bi-metallic strip you have.) Glue another small strip of wood along the top edge of one side of the base.

    Base Layout ()

  • 2 / 4

    Drive a small nail (not galvanised) in the base. Position it on the opposite end from the small wood strip.This will be the contact for the bimetallic strip, so locate it where the mounted strip can touch it when it is heated. Temporarily hold the bi-metallic strip on its support and adjust the nail height to make a gap of about 1/16 inch between the nail and the bi-metallic strip.

    Base with Contact Added ()

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    Heat the bi-metallic strip to determine the direction of the curve. Mount the bi-metallic strip to the small wood strip in a manner that allows the strip to curve down to contact the nail. You can mount the strip to the block using a small nail, screw or epoxy cement.

    Base with Nail and Bi-metallic Strip ()

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    Connect a piece of wire to the positive terminal of the battery and to the bimetallic strip. Connect a second piece of wire from the nail to one lead of the bell. Finally, connect a third wire from the negative post of the battery to the second lead of the bell.

  • Checklist

    Things you will need

    • Wood support
    • Bi-metallic strip
    • Small nails
    • Epoxy glue (optional)
    • Bell wire
    • Bell
    • Battery
  • More information

    Tips and warnings

    Buy a battery the same voltage as the bell. Six volt lantern batteries are a good choice and match well with the bell listed in the resources. Radio Shack and educational supply stores may also be a local source for the bell.

    This type of alarm is to be considered for demonstration purposes only. Do not rely on it to save your life in the event of a fire.


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