A "U" tube manometer may be filled with what substance?

A "U" tube manometer is a device used to measure pressure, typically in accordance with the principle of hydrostatic pressure differences. It consists of a U-shaped tube partially filled with a liquid. The choice of liquid used in a manometer is crucial, as it needs to have specific properties to ensure accurate and reliable measurements.

Light oils are suitable for use in a manometer because they have a relatively low density compared to other liquids like water, which allows for a more sensitive measurement of pressure changes. Furthermore, light oils have a low vapor pressure, which minimizes the risk of evaporation and bubble formation, both of which can affect accuracy in readings.

Selecting a liquid like ethyl alcohol, while plausible due to its low density, could introduce issues regarding its volatility and flammability. Nitrogen, being a gas, is unsuitable for a manometer since manometers specifically rely on liquid columns to measure pressure differences. Lastly, granular substances do not serve the purpose of measuring pressure, as they cannot fill a column to create the necessary hydrostatic pressure differential.

Thus, light oils are the most appropriate choice for filling a U tube manometer, ensuring precise pressure measurement with minimal complications.