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Pressure as a physical quantity:
pressure = force per unit area.
SI unit: pascal 1 pascal(pa) = N/m^2
somtimes measured in bars or tonnes per square inch.
Pressure in a liquid: p=hpg where h is the height of the liquid column (in metres), p is the density of the liquid and g is the acceleration due to gravity, usually 9.81 metres per second squared.
Pressure is measured using various types of manometers. Atmospheric pressure is measured using a barometer.

Pres"sure (?; 138), n. [OF., fr. L. pressura, fr. premere. See 4th Press.]

1.

The act of pressing, or the condition of being pressed; compression; a squeezing; a crushing; as, a pressure of the hand.

2.

A contrasting force or impulse of any kind; as, the pressure of poverty; the pressure of taxes; the pressure of motives on the mind; the pressure of civilization.

Where the pressure of danger was not felt.
Macaulay.

3.

Affliction; distress; grievance.

My people's pressures are grievous.
Eikon Basilike.

In the midst of his great troubles and pressures.
Atterbury.

4.

Urgency; as, the pressure of business.

5.

Impression; stamp; character impressed.

All saws of books, all forms, all pressures past.
Shak.

6. (Mech.)

The action of a force against some obstacle or opposing force; a force in the nature of a thrust, distributed over a surface, often estimated with reference to the amount upon a unit's area.

Atmospheric pressure, Center of pressure, etc. See under Atmospheric, Center, etc. --
Back pressure (Steam engine), pressure which resists the motion of the piston, as the pressure of exhaust steam which does not find free outlet. --
Fluid pressure, pressure like that exerted by a fluid. It is a thrust which is normal and equally intense in all directions around a point. Rankine. --
Pressure gauge, a gauge for indicating fluid pressure; a manometer.

Pres"sure (?), n.

Electro-motive force.

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