how is inches of mercury measured?

Answer 1

Go to this web page:

http://www.centennialofflight.gov/2003FF/pressure/index.htm

Go down to the section titled "Measuring Pressure." They have an excellent diagram. It's labeled "76 cm" instead of "29.92 in" but it will show you how to measure inches (or centimeters) of mercury.

(Note: The vacuum at the top of the mercury column is very important. You can't have any air (and air pressure) up there, or it will invalidate the reading, since you want to know the weight of mercury that the atmospheric air can support WHEN THERE IS NOTHING AT THE TOP OF THE MERCURY THAT IS PUSHING IT DOWN.)

Answer 2

Inches of mercury is just the height of the mercury column in the barometer. There is a gauge at the top of the barometer you bring down to the top of the mercury to measure the height.

It is a useless figure as it cannot be used in calculations. Internationally, air pressure is measured in hectopascals. It used to be in millibars but the bar is not a standard metric unit while the pascal is. hectopascals were shosen as the figure is the same for millibars - 1000hPa = 1000mb.