A bourdon gauge has a thin-walled metal tube, somewhat bent into the form of a C. A lever system is attached to its free end that magnifies any motion of the free end of the tube. On the gauge’s fixed end, is a fitting that is threaded into a boiler system. As the boiler pressure increases, it travels through the tube. Like the snakelike paper whistle, the metal tube starts to straighten as the pressure increases inside it. As the tube then straightens, the pointer moves around a dial which indicates the pressure in psi.