Humanities › History & Culture History of the Altimeter Measuring Distance Above Sea Level or the Ground Below Aircraft Share Flipboard Email Print Lupus in Saxonia/Wikimedia Commons/CC BY-SA 4.0 History & Culture Inventions Famous Inventions Famous Inventors Patents & Trademarks Invention Timelines Computers & The Internet American History African American History African History Ancient History and Culture Asian History European History Genealogy Latin American History Medieval & Renaissance History Military History The 20th Century Women's History View More By Mary Bellis Inventions Expert Mary Bellis covered inventions and inventors for ThoughtCo for 18 years. She is known for her independent films and documentaries, including one about Alexander Graham Bell. our editorial process Mary Bellis Updated May 15, 2019 The altimeter is an instrument that measures vertical distance with respect to a reference level. It can give the altitude of the land surface above sea level or the altitude of an airplane over the ground. French physicist Louis Paul Cailletet invented the altimeter and the high-pressure manometer. Cailletet was the first to liquefy oxygen, hydrogen, nitrogen, and air in 1877. He had been studying the composition of gases given off by iron in the blast furnace of his father's ironworks. At the same time, Swiss physician Raoul-Pierre Pictet liquefied oxygen using another method. Cailletet had an interest in aeronautics, which led to developing an altimeter to measure the altitude of an airplane. Version 2.0 AKA the Kollsman Window In 1928, a German-American inventor named Paul Kollsman changed the world of aviation with the invention of the world's first accurate barometric altimeter, which was also called the “Kollsman Window.” His altimeter converted barometric pressure into the distance above sea level in feet. It even allowed pilots to fly blind. Kollsman was born in Germany, where he studied civil engineering. He emigrated to the United States in 1923 and worked in New York as a truck driver for Pioneer Instruments Co. He formed the Kollsman Instrument Company in 1928 when Pioneer didn't accept his design. He had then-Lieutenant Jimmy Doolittle conduct a test flight with the altimeter in 1929 and was eventually able to sell them to the United States Navy. Kollsman sold his company to the Square D Company in 1940 for four million dollars. The Kollsman Instrument Company eventually became a division of Sun Chemical Corporation. Kollsman also went on to file hundreds of other patents, including those for converting salt water into fresh water and for a slip-resistant bathroom surface. He even owned one of the earliest ski areas in the United States, Snow Valley in Vermont. He married actress Baroness Julie "Luli" Deste and purchased The Enchanted Hill estate in Beverly Hills. The Radio Altimeter Lloyd Espenschied invented the first radio altimeter in 1924. Espenschied was a native of St. Louis, Missouri who graduated from the Pratt Institute with a degree in electrical engineering. He was interested in wireless and radio communications and worked for telephone and telegraph companies. He eventually became the director of high-frequency transmission development at Bell Telephone Laboratories. The principle behind how it works involves monitoring a beam of radio waves transmitted by an aircraft and their time to return as reflected from the ground to calculate altitude above the ground. The radio altimeter differs from the barometric altimeter in showing altitude above the ground below rather than above sea level. That is a critical difference for improved flight safety. In 1938, the FM radio altimeter was first demonstrated in New York by Bell Labs. In the first public display of the device, radio signals were bounced off the ground to show pilots the altitude of an aircraft. Besides the altimeter, he was also the co-creator of the coaxial cable, an important component of television and long-distance telephone service. He held over 100 patents in communications technology.