Question

what are the basic principles of manometer in short bullets

Answer 1

Manometer is a very simple device which used to measure pressures (typically atmospheric pressures). It is generally a U-shaped calibrated glass tube containing a high density liquid (typically mercury) of a known value.

Now to measure the pressure exerted by a fluid (typically gas), the fluid is allowed to exert pressure on one of the closed ends of the tube. Under the effect of pressure the liquid inside the manometer tube gets displaced and the amount of displacement is noted. This can in effect be used to measure the pressure being exerted on the tube.