Bi-metal refers to a material made from two different metals that are bonded together. This combination takes advantage of the unique properties of each metal, such as strength, conductivity, or resistance to corrosion. Bi-metal is commonly used in applications like thermometers, where the differing expansion rates of the metals can be utilized to measure temperature changes. One popular example of bi-metal is the bi-metallic strip, which consists of two metals with different thermal expansion coefficients. When heated, the strip bends due to the unequal expansion, allowing it to activate switches in devices like thermostats. This principle is also applied in various industrial and engineering applications.