Answer:
A barometer is used to measure air pressure. A Mercury Barometer uses a column of liquid mercury supported by air pressure. The length of this liquid column gives a measure of the air pressure. Air pressure is commonly expressed in terms of "millimetres of mercury" (mmHg). Mercury is considered as a hazardous material and Mercury Barometers cannot be legally manufactured in Europe as of 2007. An Aneroid Barometer traditionally uses an evacuated corrugated metal canister that expands or contracts in response to changes in air pressure and this mechanical movement is amplified to indicate the air pressure in any required units on a calibrated display. The commonest pressure units displayed are in "millibars" (mbar). The latest Aneroid Barometers use a tiny evacuated capsule made of semiconducting materials that responds electrically to changes in air pressure, enabling very precise and repeatable measurement. The electrical signals from this device are processed electronically to provide a visual display of air pressure or to be recorded on a data-logging device as required.