Which of the following instruments is used for measuring atmospheric pressure?

Step 1: Analyze the question The question asks which instrument measures atmospheric pressure, which is the force exerted by the weight of air above a given point.

Step 2: Explain the concept Each instrument is named for what it measures:

• Hydrometer: measures the density (relative density) of liquids, e.g. checking battery acid or milk purity.
• Hygrometer: measures humidity, the amount of water vapour in the air.
• Thermometer: measures temperature.
• Barometer: measures atmospheric pressure. The word comes from the Greek baros, meaning "weight" or "pressure." Common types are the mercury barometer and the aneroid barometer. Pressure is commonly given in millimetres of mercury (mmHg) or millibars (mb).

Step 3: Solve by elimination

• A is wrong because it deals with liquid density.
• B is wrong because it deals with air moisture.
• C is wrong because it deals with heat.
• D is correct because it deals with air pressure.

Step 4: Answer The correct option is D. Barometer.

Tip: Link the prefix to the meaning: Baro = pressure, Hygro = moisture, Hydro = liquid or water, Thermo = heat. Another common slip is mixing up hydrometer and hygrometer. Remember that "hygro" sounds like "humid", which helps you recall that it measures humidity.