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In early days of gas detection, measurements were made by means of heat conductance in a hot filament. During the 1950s, the heat conductance K of a gas was the easiest way to accomplish this task using the Chapman–Enskog equation. The heat-capacity measurement technique is so efficient that industry used it for decades, and scientists could not find a better way to identify gases. A hot filament detects gases used in industrial processes, that could be dangerous to humans, e.g., hydrogen, or for other gases such as natural gas or helium. It is still used to control gas flow. The other important technique invented in the fifties was gas spectrometry. Its biggest problem was the large instrumentation needed at that time to perform this kind of measurement. The hot-filament monopoly started to change when semiconductor surface studies started to give some results. Walter Houser Brattain, for instance, when receiving the Nobel Prize in Physics in 1956, highlighted in his Nobel lecture the importance of studying surface changes in germanium when in contact with several gases.1 In his words, “if surfaces are so important, what do we know about them? What is a surface? What properties does a surface have that a physicist can measure?”
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