Now you will have pumped out the hollow chamber with a vacuum pump so that there is no air in the chamber. How that is done doesn't concern us here, just accept that it can be done easily. There are sensitive ways to observe and measure how much the metal plates move in and out. That is one type of barometer. Though other liquids can be used in a barometer, mercury is the most common.
Barometers serve several very important purposes.
Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.