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INTRODUCTION
Vacuum is a Greek word that means‘ empty‘, indeed in practical sense , vacuum means an enclosure where pressure is lower than the atmospheric pressure. The atmosphere is composed of various gases mainly nitrogen-78%, oxygen 21%, carbon dioxide 0.03 % etc. Pressure exerted by these gases is given in units called Pascals, Torrs or atmospheres.
The atmospheric pressure is usually 760 Pascal’s. Existence of any pressure below these 760 Pascals is called a vacuum.
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