In the metric system, Hg stands for mercury. It is often used to denote the element mercury in chemical contexts. However, in the context of measurement, Hg is commonly used to represent the unit of pressure known as millimeters of mercury (mmHg).
Millimeters of mercury (mmHg) is a unit of pressure that is based on the height of a column of mercury. This unit is traditionally used in medicine and physiology to measure blood pressure. For example, a typical blood pressure reading might be 120/80 mmHg, where 120 mmHg is the systolic pressure and 80 mmHg is the diastolic pressure.
The use of mercury in this unit dates back to the invention of the mercury barometer by Evangelista Torricelli in 1643. The height of the mercury column in the barometer changes with atmospheric pressure, providing a reliable way to measure pressure.
In summary, Hg in the metric system primarily refers to mercury, and it is used in the unit mmHg to measure pressure, particularly in medical contexts.