The Bar and Gauge apparatus is a laboratory tool used to measure the density and specific gravity of liquids. The apparatus consists of a glass U-tube filled with the liquid to be measured and a set of weights or a float attached to a gauge arm. The U-tube is first filled with the liquid, ensuring that any air bubbles are removed.

The weights or float are then attached to the gauge arm and the assembly is placed in the U-tube. The gauge arm is adjusted until the weights or float are balanced and the density or specific gravity is read from the scale on the arm.

The Bar and Gauge apparatus is commonly used in chemical and biochemical laboratories to determine the concentration of solutions, as well as in the petroleum industry to measure the density of crude oil and other petroleum products. The accuracy and precision of the measurements depend on the design of the apparatus and the skill of the operator.