Mobile trough mercury barometer working principle: The principle of Torridelli is used: a glass tube evacuated at the top is inserted into a mercury tank. Under atmospheric pressure, the mercury column that is poured into the glass tube will maintain a certain height. At this time, the mercury column is to the mercury tank. The pressure generated on the surface balances the atmospheric pressure acting on the groove surface. If the height of the mercury column is measured with a vertically erected scale near it (as shown in the figure, the zero point of the scale is taken on the surface of the mercury cell), the shape may be As the simplest barometer, its reading is the atmospheric pressure obtained. Given the atmospheric pressure at that time is p, the height of the mercury column is H, so In the formula It is the density of mercury at temperature t and g is the local gravitational acceleration. It can be seen that under the same atmospheric pressure value, the measured value may still have different readings due to the difference in air temperature and gravitational acceleration at the measurement location of the barometer. In order to correctly reflect the atmospheric pressure, a standard state must be specified with the following qualifications: (1) Take the mercury density at 0 °C, ie =1.35951 * 10 kg ; (2) Take the standard acceleration of gravity, that is g = 9.80665 m/s In theory, any kind of liquid can be used to make barometers, but mercury has its unique advantages: (1) The density of mercury is large, so the height of the formed liquid column is appropriate. (2) The saturated vapor pressure of mercury is very small, so the additional pressure generated by the mercury vapor in the vacuum part of the top of the glass tube has a negligible effect on the accuracy of the reading; (3) After a certain process, it is easy to obtain high-purity mercury. Vanadium Battery Membrane,Vanadium Redox Battery,Vanadium Redox Flow Battery,Vanadium Battery Stack Mianyang Prochema Commercial Co.,Ltd. , https://www.gustek.com
Simple mercury barometer 
Mobile trough mercury barometer working principle