Final answer:
A mercury manometer uses Pascal's principle to measure pressure, indicating that the pressure change is transmitted undiminished throughout the enclosed fluid, allowing us to read the pressure difference in units like Pascal (Pa), torr, or bar.
Step-by-step explanation:
The mercury manometer relies on the principle that when a fluid is subjected to increased pressure, it reacts to the difference in pressure between two points within the fluid. The correct principle that a mercury manometer relies on to measure pressure inside an air duct is (a) Pascal's principle. Pascal's Principle states that a change in pressure applied to an enclosed fluid is transmitted undiminished to all portions of the fluid and to the walls of its container.
The SI unit of pressure is the Pascal (Pa), although other units like torr, bar, and mm Hg are also commonly used. In the case of measuring gas pressure with a manometer, the difference in mercury levels after a change in pressure gives us a reading which can be converted to various units of pressure, such as torr, Pa, or bar.