Explanation:

A U-tube manometer is a device used to measure the pressure difference between two points in a fluid. It consists of a U-shaped tube partially filled with a liquid, typically mercury or water. The pressure difference is determined by the difference in height of the liquid columns in the two arms of the U-tube.

Reasons for using an inverted U-tube manometer:

1. Small pressure differential: When the pressure differential is small, it is more convenient to use an inverted U-tube manometer. In this configuration, the fluid being measured is connected to one arm of the U-tube, while the other arm is open to the atmosphere. The pressure difference causes the liquid column to move up or down in the two arms, and the difference in height can be measured to determine the pressure difference.

2. Greater sensitivity: An inverted U-tube manometer is more sensitive to small pressure differentials compared to an upright U-tube manometer. This is because the height difference between the liquid columns is maximized in the inverted configuration, allowing for more accurate measurements.

3. Reduced errors: Using an inverted U-tube manometer minimizes errors due to parallax and meniscus effects. Parallax occurs when the observer's line of sight is not perpendicular to the liquid column, leading to inaccurate readings. In an inverted U-tube manometer, the observer can directly read the height difference without any parallax errors. Meniscus effects, such as the curvature of the liquid surface, are also reduced in the inverted configuration.

4. Convenience: Inverted U-tube manometers are easier to read and operate compared to upright U-tube manometers. The liquid column moves in a direction that is visually more intuitive, making it easier to determine the pressure difference.

In summary, an inverted U-tube manometer is preferred when the pressure differential is small because it offers greater sensitivity, reduces errors, and provides convenience in reading and operation.