Thermalstat question

當受熱果陣, 個strip 環左去其中一邊, 點到個electricity supply, 咁咪有電過囉, 有電過個alarm就會響 =)
thats why a fire alarm has this kind of system inside itself=)

A bimetallic strip composes of two different metals, eg. copper and iron, welded together. Becuase of the different expansitivity of the two metals, copper expands more than iron at the same rise in temperature. As a result, the strip will be bent due to the difference in lengths of the two metals after expansion.

When the bimetallic strip is used as a thermostat(恆溫器), a rise in temperature will make the strip to bend. The bending of the strip could be made to function as a switch to close (or open) an electric circuit. When the temperature falls, the strip cools down and the two metals contract, returing to the original straight form thus cutting off (or turning on) current in the curcuit.

Using the bimetallic strip in an oven could maintain the oven temperature not to exceed a preset value, thus serving the purpose of keeping the oven at that particular constant temperature. This is the function of a thermostat.

Another example of the bimetallic strip is used in fire alarm, when temprature rises due to fire, the bimetallic strip gets hot and expand. The bending of the strip closes a fire alarm circuit, hence sounding out an alarm.

You may refer to the following web-page for more information:

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bimet.html