An instrument measuring atmospheric pressure. A mercury barometer compares the hydrostatic pressure of a measured mercury column with the pressure above it. Its equation is , with the vertical separation of the liquid levels.
A measured mercury column represents pressure through its actual mass density and local gravitational acceleration. An equivalent reference column therefore obeys . If the indicated scale length expands as and mercury volume as , the first-order thermal factor multiplying the scale reading is . These factors describe a pressure-unit reduction, not a physical change in local gravity.[ref]
Mercury Barometers and Manometers by W. G. Brombacher and others. §§9–10 and equation45
A mercury barometer with its lower end in a reservoir exposed to atmospheric pressure. A fixed-cistern scale must allow for the changing reservoir level, while an adjustable cistern can restore a fixed lower reference before each reading.
An adjustable-cistern barometer associated with Jean Nicolas Fortin. Restoring the cistern surface to a fixed index makes the lower mercury level reproducible before the upper level is read.[ref]
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A barometer is an instrument used to measure atmospheric pressure. It helps in forecasting weather and determining altitude. There are two main types of barometers: 1. **Mercury Barometer**: This consists of a glass tube filled with mercury, which is inverted into a dish of mercury. As atmospheric pressure changes, it causes the mercury level in the tube to rise or fall. The height of the mercury column is measured in millimeters or inches and corresponds to the atmospheric pressure.