What type of barometers are used in aeroplanes and why?

The altimeter in an aircraft works the same as an aneroid barometer. Pressure differences with height act on a spring and diaphragm housing, which moves the needle around. There is a subscale and adjusting nut, which the pilot uses to set the sea level pressure for the area. The atmospheric pressure drops about 1 millibar every 30 feet. So if the altimeter subscale is set while on the ground, then the aircraft takes off, and climbs to 1000 feet, the pressure will be about 33 mbar less than on the ground, and the height indicating needle will read 1000.