An aneroid barometer measures atmospheric pressure by balancing which of the following?

• A. The weight of the air above it against the spring action of the aneroid.
• B. Electrical resistance of a sealed capsule.
• C. The temperature of the surrounding air.
• D. The density of the air.

Answer: (A)

Aneroid barometers measure pressure by balancing the external atmospheric force against a restoring force inside the instrument. The atmosphere presses on a sealed, flexible metal chamber (the aneroid cell), tending to compress or expand it as pressure changes. This movement is transmitted through a linkage to a pointer, while a calibrated spring provides the counteracting force. The point where these forces balance corresponds to the current atmospheric pressure, which is read on the dial. The idea that the external air’s weight is opposed by the spring captures why the measurement reflects pressure. Electrical resistance, temperature, or air density are not how the device directly measures pressure, so they don’t determine the reading.