A manometer is an instrument used for measuring

Analyze the question: The question asks what a manometer measures. It is a definition question, so you need to know the instrument.

Explain the concept: A manometer is a U-shaped tube containing a liquid (usually mercury or water). One end is connected to the gas or liquid whose pressure is to be measured, and the other end is open to the atmosphere or sealed. The difference in the liquid levels between the two arms gives the pressure difference:

P = ρgh

where ρ is the density of the manometer liquid, g is the acceleration due to gravity, and h is the difference in height.

Go through the options:

• A. Relative density of liquid: This is measured with a hydrometer or a density bottle, so it is wrong.
• B. Pressure in liquids and gases: A manometer measures the pressure of a fluid (gas or liquid) relative to the atmosphere or another reference. This is correct.
• C. Humidity: This is measured with a hygrometer, so it is wrong.
• D. Atmospheric pressure: This is measured with a barometer. A manometer is not mainly meant for this, so it is wrong.

My own working agrees with it.

Exam tip: Match each instrument to what it measures:

• Barometer measures atmospheric pressure.
• Manometer measures fluid (gas or liquid) pressure.
• Hydrometer measures relative density of liquids.
• Hygrometer measures humidity.

"Mano" is linked to pressure, so remember "Manometer = pressure of fluids". The barometer is the one tied to the atmosphere, which is how you tell option D from option B.