The relationship between atmospheric pressure and height above sea level was known by Blaise Pascal and Edmond Halley in the mid-seventeenth century: one can be used to measure the other. In 1862 James Glaisher used an aneroid barometer as a height instrument on a 7-mile (11 km) high balloon flight.
The barometric (or pressure) altimeter uses an aneroid barometer to register pressure change. The higher you go, the less air there is above pressing down on the instrument. Near sea level, under normal conditions, the pressure changes by one millibar (0.03 inches of mercury) for every 27 feet (8 m) of altitude. Alas, the pressure also changes with the weather and the temperature. In addition, the pressure-height relationship is not truly linear but exponential.
Paul Kollsman (1900-1982), a German mechanical engineer, emigrated to the United States in 1923. His watchword was "accuracy." Previous altimeters determined airplane heights to a few hundred feet. This was of no problem in perfect weather and daylight, but made flying in fog or clouds or at night rather too adventurous. Kollsman's instruments, when correctly set, measured height to within a few feet.
In 1929 army flyer 1st Lieutenant Jimmy Doolittle used the barometric altimeter, together with ground- based radio navigation systems and gyroscopic artificial horizons and compasses, to fly 15 miles (24 km) "by the gauges." These successful "instrument flights" ensured that few of the world's planes flew without a Kollsman altimeter thereafter. The ground pressure at the airport a flight is approaching is still referred to as the Kollsman number.
