Definition
A thermometer that measures temperature using a sensing element made of two different metals bonded together. Because the two metals expand at different rates when heated, the bonded strip bends as temperature changes, and that movement is mechanically linked to a needle on a dial to display the temperature. In light aircraft it is commonly used as the Outside Air Temperature (OAT) gauge, with the sensing probe protruding through the windshield or fuselage into the airstream.
Plain English
A simple temperature gauge that works by using two metals stuck together. The metals bend as they warm or cool, and that bending moves a needle to show the temperature.
Context Anchor
Seen in discussions of the outside air temperature gauge on an aircraft instrument panel.
Derivation
Bi- means 'two' and metallic means 'made of metal,' so a bimetallic strip is literally a strip made of two metals. Knowing this tells you how the thermometer works: the two metals expand by different amounts when heated, so the strip bends, and that bending drives the needle.
Why Pilots Care
Supplies the accurate outside air temperature reading required for density altitude calculations, performance planning, and icing risk assessment.
Analogy
Think of two thin strips glued together that do not grow by the same amount when heated. As one side changes size more than the other, the joined strip bends, and that bend can move a needle.
Intuition Check
Do not assume this thermometer must be electrical. A bimetallic-type thermometer can work by simple metal bending caused by temperature change.
Example Sentence 1
Before takeoff, the pilot glanced at the bimetallic-type thermometer mounted in the windshield and noted the outside air temperature for the density altitude calculation.
Example Sentence 2
During the climb the bimetallic-type thermometer showed a steady drop in temperature as the aircraft gained altitude.