An instrument used to measure atmospheric pressure is called a

Barometer is an instrument used to measure the atmospheric pressure.

Explanation:

Torricelli did an experiment by filling a one meter long test tube completely with mercury. He took a beaker with some amount of mercury in it and inverted the test tube filled with mercury by closing its open end into the mercury of beaker ensuring that no bubble is trapped into the mercury of test tube during the process. This experiment was performed at normal temperature and pressure. Now some of the mercury from the test tube fell down into the beaker through the open end until the level becomes stagnant and this denotes the atmospheric pressure in terms of mercury column. There was a vacuum created at the top of the test tube called torricellian vacuum which holds back the remaining mercury height and the rest of the mercury level in the beaker is facing the atmospheric pressure.

the instrument used to measure the atmospheric pressure is called barometer.

It consists of a long tube open at one end and inverted into mercury. The normal pressure is 1 atm.

1 atm = 76 cm of mercury

1 atm = 1.01 x 10^5 Pa