Tale of the Thermometer

The ability to measure temperature became important in many industries in the 1800’s. But how could one measure temperature? Scientists still doesn’t understand what caused heat, and the idea that temperature was a measure of the speed with which atoms move was not universally accepted.

Early systems tapped the thermal expansion of liquids stored in narrow tubes as a way to measure temperature, writes Jeremy Webb in his book, Nothing: Surprising Insights Everywhere from Zero to Oblivion. The level of the liquid was marked at two “fixed temperatures”, such as the melting and freezing temperatures of water. And everything in between was marked as intermediate values.

But this created a 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.”

It was in the 1840’s that a French scientist, Henri Victor Regnault, found a way out. He 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. How did he prove that?

“Different designs of air thermometer calibrated at the freezing and boiling points of water gave consistent estimates of temperatures.”

So yes, the air thermometer was a “true” measure of temperature. But it was hard to use. So it became the way to instead calibrate other more practical thermometers instead.

Ironically, people could now measure temperature without still knowing what exactly they were measuring!