The Bourdon gauge is shown in figure 9-4. Itworks on the same principle as that of the snakelike,paper party whistle you get at a New Year party,which straightens when you blow into it.Within the Bourdon gauge is a thin-walled metaltube, somewhat flattened and bent into the form of aC. Attached to its free end is a lever system thatmagnifies any motion of the free end of the tube. Onthe fixed end of the gauge is a fitting you thread intoa boiler system. As pressure increases within theboiler, it travels through the tube. Like the snakelikepaper whistle, the metal tube begins to straighten asthe pressure increases inside of it. As the tubestraightens, the pointer moves around a dial thatindicates the pressure in psi.