The automatic fire alarm system uses the fact that different solids expand by different amounts on heating . The alarm contains two metal stripes , one of iron and the other of brass , firmly bolted together . Brass expands more than iron on heating . Therefore , when this bimetallic strip gets heated because of fire , it bends on the side of iron . As it bends , the circuit of the electric bell is completed . The starts ringing to give the alarm . As the fire is put out , the strip gets cooled and straightens again . This causes the circuit to break , thus switching off the alarm .
please feel free to comment/resteem/follow 