how to build a thermometer -- 5/08/19
Today's selection -- from Nothing by Jeremy Webb. Scientists in the early 1800s wrestled with the problem of measuring temperature, often using brandy in a glass tube. It was a time when they did not understand what caused heat, and the idea that temperature was a measure of the speed with which atoms move was flatly rejected by London's Royal Society:
"Early efforts at measuring temperature were purely empirical. There existed a standardized method -- within each laboratory at least -- for determining 'degrees of heat' in a reproducible manner. The most useful thermometers exploited the thermal expansion of liquids constrained in glass bulbs and narrow tubes. The level of the liquid was marked at two 'fixed temperatures,' such as the melting and freezing temperatures of water. Then, unknown temperatures were measured as 'degrees of heat' that were etched as a scale between the two fixed points.
"The biggest problem for early workers was a 'thermal catch-22.' The scale-marking process assumes that the liquid expands an equal amount for every unit rise in temperature. But this assumption cannot be verified unless one measures the thermal expansion of the liquid, and to do that one requires ... a thermometer.
"By the early 19th century, no solution to this circularity was in sight. Instead, different workers simply asserted that one thermometer or another was better than the others. Early thermometers used 'spirit' -- essentially brandy -- and this was generally inferior to mercury. However, exhaustive comparisons in the 1840s by French scientist Henri Victor Regnault showed that an 'air thermometer' -- which measures changes in pressure of dry air in a sealed container -- was superior to both in its reproducibility and inter-comparability.
|Various thermometers from the 19th century.|
"Different designs of air thermometer calibrated at the freezing and boiling points of water gave consistent estimates of temperatures. In contrast, liquid-in-glass thermometers varied in their performance depending on the properties of glass, and the type of liquid. Slowly the air thermometer, which was difficult to use, began to be viewed as definitive and was used to calibrate other, more practical thermometers.
"Crude as early measurements were, they brought some order to the thermal world. Reproducible readings aided everything from cooking to industrial processes. But still no one really knew what it was they were measuring! ...
"It is hard to imagine a time when even the greatest scientific pioneers did not understand that everyday objects are made of atoms, that heat is the kinetic energy of moving atoms, and that temperature is a measure of the speed with which atoms move -- specifically the square of the average molecular speed. Although ideas of this kind were advanced by the likes of John Herapath in 1820 and John James Waterson in 1845, they were roundly rejected by London's Royal Society."