The principle of the mercury barometer was discovered by the Italian physicist Evangelista Torricelli in about 1643. That principle can be illustrated in the following manner. A long glass tube is sealed at one end and then filled with liquid mercury metal. The filled tube is then inverted and its open end inserted into a bowl of mercury. When this happens, a small amount of mercury metal runs out of the tube, leaving a vacuum at the top of the tube.
Under normal circumstances, the column of mercury in the glass tube stands at a height of about 30 in (76 cm). The column is sustained because air pressure pushes down on the surface of the mercury in the bowl at the bottom of the barometer. At the same time, the vacuum at the top of the glass tube exerts essentially no pressure on the column of mercury. The height of the mercury column in the glass tube, then, reflects the total pressure exerted by the atmosphere at the moment of measurement.
In theory, a barometer could be made of any liquid whatsoever. Mercury is chosen, however, for a number of reasons. In the first place, it is so dense that the column sustained by air pressure is of practicable height. A similar barometer made of water, in comparison, would have to be more than 34 ft (100 m) high. Also, mercury has a low vapor pressure and does not, therefore, evaporate easily. In a water barometer, the situation would be very different. Water has a much greater vapor pressure, and one would have to take into consideration the pressure exerted by water vapor at the top of the barometer, a factor of almost no consequence with a mercury barometer.
Two important additions needed to increase the accuracy of a barometer are a vernier scale and a thermometer. The vernier allows one to make an even more accurate measurement than is possible by reading the scale itself. The thermometer is needed because the density of mercury and other materials used in the construction of a barometer change with temperature. Most barometers come equipped with thermometers attached to them, therefore, along with conversion charts that permit one to correct barometer readings for a range of actual temperatures.