THE FUSE

When your child has a fever, you reach for a thermometer and trust the number. That trust is newer than it seems. Galileo's early instrument, built around 1600, had no scale at all. It only showed that something had become warmer or colder. Later makers filled their tubes with alcohol, mercury, or air and marked them against different reference points, so two thermometers in the same room could disagree. As the historian Hasok Chang describes in Inventing Temperature, it took well over a century of comparing instruments against one another before readings from different devices meant the same thing.