Principle

• Under normal temperature and pressure (i.e., 59 degree F and 760 mmHg), the mercury column in the glass tube stays erect at the height of 30 inches (or about 76 cm). Because of the vacuum created at the top of the barometer, the mercury column does not experience pressure from the top end of the glass tube. So, regardless of the height, the mercury column rises in the glass tube because of the effect of atmospheric pressure on the mercury surface filled in a dish at the bottom of the glass tube. Thus, the height of the mercury column in the glass tube reflects the total atmospheric pressure at a particular place.
  
  

Vacuum Pump

• A vacuum pump is a tool for filling a mercury barometer with mercury. The vacuum pump partially exhausts the space within the glass tube, resulting in a forced vacuum. The mercury is then drawn into the glass tube from the mercury-filled dish at the bottom of the glass tube through the suction force produced by the empty space inside the barometer. To maintain the vacuum, the valve is closed after removing the vacuum pump.
  
  

Shape

• One of the unique features of a mercury barometer is that the level of mercury does not alter with the change in shape or volume of the barometer. This is because the level of mercury in the glass tube is only dependent on the atmospheric pressure. Mercury barometers are available in three different shapes.
  
  

Additions

• To increase the overall accuracy of a mercury barometer, one can add two other pieces of equipment to a barometer. One is the verneir scale, which can help in reading the barometric measurement with more accuracy than is possible with an ordinary barometer scale. The second is a thermometer, which is used to factor out any errors because of changes in the density of the mercury and other structural materials of a barometer. Along with a thermometer a conversion chart is also needed to correct barometer readings.
  
  

Why Only Mercury?

• Although a barometer can be made with any liquid, there are some properties of mercury that make it the ideal candidate for a barometer. It is denser than most other liquids and has a much lower vapor pressure than that of water. Mercury is also easily available.