A Bourdon pressure gauge works by measuring the amount of change in a coiled or semicircular metal tube by a pressurized fluid inside. This is due to the principle that a flattened tube tends to regain its circular form when pressurized.
The change in cross section of a pressurized tube can be minuscule and can then be covered up by the relative elasticity of easily worked materials. In order to counteract this, the tube is often coiled into a "C" shape or a helix to magnify the effects of the pressure, thus rendering it measurable. Patented in 1849 by Edward Bourdon, the Bourdon pressure gauge was lauded for its accuracy, sensitivity, and linearity relative to other methods of measuring pressure.