Pilots uses aneroid barometer to know the height above sea level because

Analysis

The question asks for the primary reason why pilots prefer the aneroid barometer over other types of barometers (like the mercury barometer) for measuring altitude in an aircraft.

Concept Explanation

An aneroid barometer is a device used to measure atmospheric pressure without using any liquid. It consists of a flexible metal box (an aneroid cell) that expands or contracts as the external air pressure changes. This movement is transmitted through a mechanical linkage to a pointer on a dial.

In aviation, this device is calibrated to read altitude instead of pressure, and it is known as an altimeter.

• Mercury barometers are tall, fragile, and contain toxic liquid mercury, making them completely unsuitable for a moving aircraft.
• Aneroid barometers are compact, portable, and do not rely on gravity to keep a liquid column level, which is essential when a plane is banking, climbing, or descending.

Step-by-Step Solution

1. Evaluate Option A: While aneroid barometers are useful, they are generally less accurate than mercury barometers, which are considered the standard for precision. Therefore, accuracy is not the main reason they are chosen for flight.
2. Evaluate Option B: Because they are compact, lightweight, and contain no liquid, they are highly portable and convenient to install in the tight space of an aircraft cockpit. This makes them the practical choice for aviation.
3. Evaluate Option C: Being "heavy and huge" would be a disadvantage for an aircraft, where weight reduction is a priority.
4. Evaluate Option D: Cost is not the primary functional reason for their use in aviation; their physical suitability is.

Conclusion

The aneroid barometer is the standard choice for aircraft because its design is compact, robust, and does not require a liquid column, making it small and convenient for cockpit use.

Exam Tip: Whenever you see questions about barometers in Physics, remember: Mercury = High accuracy but fragile/bulky; Aneroid = Portable, convenient, and ideal for mobile applications like aviation or mountain climbing.