"bimetallic strip"
by cm
Bimetallic Thermostat
Public DomainBimetallic Thermostat - Credit: Rogilbert, Wikimedia
Memorial Plaque
Public DomainMemorial Plaque - Credit: Peter R Hastings
Because different metals expand at different rates when heated, if two strips of different metals (usually steel and copper) are welded or riveted together, the combined strip will bend as it warms.   A coiled bimetallic strip is the fundamental mechanism in thermostats.  

The memorial plaque to John Harrison in Westminster Abbey incorporates a meridian line formed from two metals to represent this essential invention.