 Vacuums have a widespread use of commercial applications, including its use in welding applications, vacuum pumps, plastic molding, aerospace flight instrumentation, semiconductors, medical equipment, and finish and coating applications. A vacuum can be thought of as negative air pressure. Atmospheric pressure is defined as the weight of the atmosphere, or air, exerted on the Earth's surface and is typically measured with a barometer. At sea level, all of the weight of the atmosphere above is pressing on the Earth, therefore the atmospheric pressure is at its highest. As elevation increases, the total amount of atmosphere directly above the surface is much less than at sea level, therefore the atmospheric pressure is much lower. Atmospheric pressure decreases as elevation increases. The maximum force available from a vacuum is dependent on the barometric pressure at its location. At sea level, the barometric pressure is about 14.7 pounds per square inch under standard conditions. A vacuum can be thought of as negative air pressure and is measured with any set of force per area units. In the customary US system of units, atmospheric pressure and vacuum pump force are measured in inches of mercury, abbreviated HG, which is the atomic symbol of mercury. Determination of this measurement is made with the use of a mercury barometer. Mercury barometers are made with a long tube closed on the top end, open at the bottom, and immersed in a pool of mercury. The tube is then raised to a standing position without allowing air inside. The mercury level falls until a balance is reached between the weight of the mercury in the column and the atmospheric pressure against the surface of the mercury pool. With this method the empty space at the top of the tube forms a perfect vacuum. At sea level, the elevation of mercury will stand at about 29.9 inches of mercury. As the atmospheric pressure decreases, the height of the mercury in the tube decreases. As the atmospheric pressure increases, the height of the mercury in the tube increases. If a vacuum pump is connected to the top of the mercury column and allowed to run, the mercury column would not rise as high as with a sealed barometer. This is because no vacuum pump can achieve a perfect vacuum. Vacuum pumps are given their rating based on how high they can pull the level of mercury upward towards a perfect vacuum. Vacuum technology has developed significantly over the past few decades. Vacuum packing is used in a variety of industries to package and protect products. Plastic forming is also a widespread use of vacuum technology. In this application a sheet of plastic is heated until it is malleable. Then a mold is applied and a vacuum is used to form the plastic to the mold. Additionally, vacuum chucking is a commonly used method to hold workpieces in place without the risk of damaging their surface. In this application a piece of wood is held to a lathe so that it can be turned. New applications for the use of vacuum power will continue to grow as technology advances.