What is the working principle of the instrument (mercury) for measuring blood pressure?

The commonly used sphygmomanometer has mercury column.

Working principle of mercury sphygmomanometer: The main components of mercury sphygmomanometer are air bag, rubber ball for pressurization, mercury column and stethoscope. Under normal circumstances, the chest radiograph of stethoscope is placed on large blood vessels (such as brachial artery), and the sound produced by blood flow in blood vessels when the heart beats regularly can not be heard. In fact, blood pressure measurement is to measure the lateral pressure of large blood vessels (usually brachial artery) on surrounding tissues, that is, the pressure of blood vessels on the pressure balloon used. Take someone's blood pressure as 120/80 mmhg. When the rubber ball is gradually inflated, the pressure in the airbag gradually increases. When the pressure in the balloon is above 120mmHg (which can be read from the mercury column), there is no blood flow in the blood vessel, and the stethoscope cannot hear the sound generated by the blood flow in the blood vessel. When the blood pressure is between 120-80 mmHg, rhythmic sounds can be heard on the related large blood vessels, which is caused by the friction between the blood flowing in the blood vessels and the blood vessel wall. When the blood pressure drops below 80mmHg, the blood flowing in blood vessels can flow smoothly, so you won't hear the sound of blood rubbing against blood vessels. When the mercury column reads 120mmHg, the blood pressure is called systolic blood pressure. When the mercury column reads 80mmHg, the blood pressure is called diastolic pressure.