From AMS Glossary
A closed curved tube of elliptical cross section used in some thermometers and barometers.
The Bourdon-tube thermometer consists of a Bourdon tube completely filled with liquid. The expansion of the liquid due to a temperature change causes an increase in the radius of curvature of the tube. The curvature may then be measured by the travel of the tip of the tube. The Bourdon- tube barometer consists of an evacuated Bourdon tube and operates in a similar manner. In both cases the curvature is a measure of the difference between the pressure inside the tube and that outside.