تعريف الضغط الجوي
هو وزن عمود من الهواء مساحة مقطعه وحدة المساحات بارتفاع يعادل سمك الغلاف الجوي

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface.

كيفية قياس الضغط الجوي

لقياس الضغط الجوي نستخدم البارومتر الزئبقي الذي اخترعه تورشيلي

Torricelli

In this experiment, Torricelli used mercury which is more than 13 times as heavy as water.
A water barometer would require a tube more than 30 feet long and the water, of course, would freeze at winter temperatures.
Furthermore, water and most other common liquids release vapor which would decrease the vacuum at the top of the tube, whereas cold mercury releases very little vapor.
A glass tube, sealed at one end, is filled with mercury. Then a glass dish is partially filled with mercury.
Cover the open end of the tube with a finger, invert it, and plunge it into the mercury in the dish.
Mercury will then run out of the tube into the dish until the weight of the column of mercury in the tube exactly balances the outside air pressure.
Under standard conditions at sea level, the height will be 29.92 inch or 76 centimetres.
This experiment is important in science and technology history.
The first steam engines (Papin, Newcomen) were "atmospheric engines".
The active stroke of these engines was made by the atmospheric pressure.

في هذه التجربة قام تورشيلي بملء انبوبة زجاجية طولها واحد متر بالزئبق بعد أن طرد الهواء منها تماما أغلق طرفها المفتوح بأصبع اليد

بعد ذلك نكس الأنبوبة في حوض به زئبق بحيث تكون الأنبوبة رأسية

أبعد اصبع اليد بحيث يكون طرف الأنبوبة المفتوح مغمورا في الزئبق

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