What did old barometers consist of?

Old barometers primarily consisted of mercury in a glass tube. This design exploits the high density of mercury, which allows it to provide an accurate measurement of atmospheric pressure. When atmospheric pressure exerts force on the surface of mercury in the reservoir, it causes the mercury level in the tube to rise or fall. The height of the mercury column can be calibrated to indicate pressure in units such as millimeters of mercury.

The use of mercury is significant because its weight allows even small changes in atmospheric pressure to be measured precisely. The glass tube is sealed at one end, creating a vacuum that allows the mercury to respond to external pressure changes without the interference of air. This construction leads to reliable readings, which is essential for weather forecasting and other scientific applications.

Other options listed, such as water, oil, or alcohol, do not provide the same level of accuracy for barometric measurements. Water, for example, would require a vastly larger column to measure the same pressure due to its lower density. Oil and alcohol, while having unique viscoelastic properties, do not possess the necessary density or consistent behavior under atmospheric pressure changes to serve effectively as barometric fluids. Thus, the choice of mercury as the fluid in barometers remains pivotal for its functional reliability in measuring