Which instrument is described as measuring the absolute atmospheric pressure by balancing the air's weight against a spring action within the device?

• A. Aneroid
• B. Barometer
• C. Aneroid barometer
• D. Altimeter

Answer: (C)

The instrument described is an aneroid barometer. It uses a sealed, evacuated metal chamber that deforms as external air pressure presses on it. The weight of the air acting on this flexible chamber is balanced by a calibrated spring inside the device. This balance converts the pressure into motion that drives a pointer, giving a pressure reading without any liquid involved. Because the sensing element relies on the pressure acting against a vacuum inside the chamber and a spring, it directly measures the absolute atmospheric pressure, not a liquid-column gauge. That’s why this description fits aneroid barometers best. The other options don’t match this mechanism: a barometer with a liquid column isn’t balancing air against a spring, an “anerio d” alone lacks the barometer context, and an altimeter uses the same sensing principle for altitude but is described by its use, not the sensing method itself.