Final answer:
A manometer is a device used to measure gas pressure in a closed system, using mercury in a U-shaped tube to determine pressure differentials.
Step-by-step explanation:
The device in the image is called a manometer; it is used for measuring gas pressure in a closed system. A manometer is a U-shaped glass tube partially filled with a nonvolatile liquid, often mercury. One end of the U-tube is connected to the gas chamber where the pressure measurements are required, and the other end is either closed, known as a closed-ended manometer, or open to the atmosphere, known as an open-ended manometer. The difference in height between the mercury levels on each side of the manometer equates to the pressure differential and is used to calculate the gas pressure in the system. Manometers are distinct from barometers, as the latter are specifically designed for measuring atmospheric pressure.