Where Does a Doorknob Get Its Cold From… And Whose Heat Is It?

In this calming long-form documentary the subject is a brass doorknob in a dark hallway. It is cold — and where that cold came from is not a childish question. The answer took three centuries: Joseph Black separating heat from temperature, Rumford boring cannon, Fourier's law of conduction in 1822, Carnot on the motive power of fire, Joule's paddle wheel, and Thomson and Clausius. There is no cold in the knob and there never was. Cold is not a substance. What you felt was your own heat leaving you. Let that settle. Sources: Fourier, Théorie analytique de la chaleur (1822); Carslaw and Jaeger, Conduction of Heat in Solids (1959).