The earliest barometers consisted of a long tube in a dish of mercury. Air pressing down would force the mercury from the dish to the tube to a height of approximately 30 inches at sea level. More recently, aneroid barometers have been developed. These instruments consist of a compartment from which some of the air has been removed. This compartment expands or contracts with changes in atmospheric pressure, causing a needle to move to and fro.
All aneroid barometers have an adjustment screw in the back.
Adjust the set screw so the barometer reads as close as possible to this value. Don't be concerned about the exact reading or the effect of elevation. It's the changes in pressure that are important in relation to weather changes. Atmospheric pressure varies little between indoors and outside, so this instrument can and should be kept indoors. An instrument used to measure precipitation.