History and Application of the Gas Law

Another unit of pressure arises from the importance of the atmosphere. The average pressure at sea level is close to 760 torr. The common observation of an apparent average atmospheric pressure led to the definition of a unit called an atmosphere (atm), which was set equal to 760 torr. As science has become more precise in measuring it, this equivalence still remains  

But the definition of the unit atm has since been changed to reflect SI units. It is now defined in terms of the pascal  

Combining the last two relationships gives us a formal definition of the torr as well  

The three units that we will use most often in this book are the atm, torr, and kPa. In some engineering fields, you will also be likely to encounter other units, including pounds per square inch. None of these should cause you particular difficulty as long as you are careful that any calculations you do properly account for 

The ton- is named after Evangelista Torricelli (1608—1647), who was the first person to sug t building a mercury barometer. 

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