What type of instrument is used to measure atmospheric pressure?
There are a few instruments one can use.
One, a more common device, is called a mercury barometer:
In a mercury barometer, a tube filled with mercury is inverted into a dish also containing mercury (one must be careful not to get any air in the tube!).
After the tube is placed in the dish, some of the mercury in the tube flows into the dish mercury, but most stays in the tube.
It flows until the atmospheric pressure equals the pressure at the base of the tube due to Earth's gravity.
So, simultaneously, the atmospheric pressure is pushing down on the dish, causing mercury to go up the tube, and gravity is causing the mercury to flow down the tube.
The pressure is measured by the height of the mercury column in the tube. Standard atmospheric pressure is defined (at least to some extent) as the pressure sufficient to support a column of mercury exactly
Another device one can use (a more practical one at that) is the famous Bourdon pressure gauge:
This device you can use if you don't have access to mercury...
Inside this device is a C-shaped tube that can uncoil/straighten when subjected to pressure changes. It's kind of a like a clock, in the way you can easily tell what the pressure is at any place and time.
This particular gauge measures pressure in units of bars; the SI unit of pressure is the atmosphere (