The part labelled J is called

The part labelled J is most likely a manometer. A manometer is an instrument used for measuring pressure, particularly in gases. It consists of a U-shaped tube filled with a liquid, such as mercury or water, and the pressure difference between two sources is measured by the difference in height between the two arms of the tube. A porosimeter is used to measure the pore size distribution of a porous material, such as soil or rock. A potometer is a device used to measure the rate of water uptake by a plant, which is important for studying plant physiology and water use efficiency. An auxanometer is used to measure the rate of growth of a plant or part of a plant, typically by measuring the movement of a lever or other indicator as the plant grows. Based on the context of the question and the options given, the part labelled J is most likely a manometer, as it is being used to measure pressure.