Pressure is force per unit area.
Demonstration of Pressure
Otto von Guericke invented the first air pump and used it to study the phenomenon of vacuum. His demonstrations of air pressure earned him a lifetime pension! The strength of air pressure was shown through his famous "horses" demonstration: teams of horses (up to 24 horses) would attempt to pull "Magdeburg Hemispheres" (Copper spheres) apart which were held together by the force of atmospheric pressure created using his vacuum pump.<ref>About.com: Inventors</ref>
Atmospheric pressure is measured by a barometer. <ref>Zumdahl, S.S., Zumdahl, S. L., DeCoste, D. J. (2006). World of Chemistry. Houghton Mifflin Company.</ref> The barometer was developed by Evangelista Torricelli after creating a sustained vacuum. In 1643 he proposed an experiment that demonstrated that atmospheric pressure determines the height to which a fluid will rise in a tube inverted over the same liquid. <ref>University of St Andrews</ref>
Units of Pressure
The standard SI unit for pressure is the pascal (Pa). Other units include atmospheres (atm), torr, and bar. "The most commonly used units for pressure are based oon the height of the mercury column (in millimeters) that gas pressure can support" <ref>Zumdahl, S.S., Zumdahl, S. L., DeCoste, D. J. (2006). World of Chemistry. Houghton Mifflin Company.(p. 401)</ref>
A related unit is the standard atmosphere (atm).
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