= Kapitza thermal boundary experiment
{c}
{title2=1941}
<Pyotr Leonidovich Kapitza>'s 1941 study suspended cleaned, annealed <copper> blocks in liquid <helium>. Sealed, gas-filled cavities held a heater and a <bronze> <resistance thermometer>. Electrical readout tracked the heating-induced <temperature> rise; the earlier readout method assumed a stable bath without a separate bath <thermometer>. A nearby unheated block warmed much less, localizing the drop near the heated surface. Gold plating and coatings tested surface effects; tight cavities prevented liquid ingress. These were solid-body readings, not probes resolving the microscopic interfacial layer. <The Study of Heat Transfer in Helium II by Pyotr Leonidovich Kapitza>[The Study of Heat Transfer in Helium II, chapter 38, §2 pp. 584–585; §6 pp. 616–621, Figs. 25–29]{ref}
\Image[/kapitza-interface-heat-flow-readout.png]
{title=<Heat flux> crossing a <thermal boundary resistance>}
{id=kapitza-interface-heat-flow-readout}
{description=Original functional schematic and invented <temperature> profile. $T_s-T_l$ denotes the interface drop after bulk corrections; no apparatus geometry, microscopic probe placement or measured trace is reconstructed. The liquid-side reference in this operational diagram is distinct from a claim that Kapitza installed a probe immediately next to the surface.}
{source=kapitza-interface-heat-flow-readout.py}
The measurement logic suggests a modern control sequence: calibrate both <temperature> readouts, establish their zero-load difference, increase heater <power>, wait for a <steady state>, and repeat while monitoring the bath. Determine lead losses and the bulk <temperature gradients> before using $r_B=(T_s-T_l)/q$. These are proposed controls derived from the <energy> balance, rather than additional reported steps in the 1941 apparatus. A nearby unheated solid also has its own interface: its reading need not equal the liquid's local <temperature> exactly. The experiment supports a drop concentrated near the surface, while a microscopic thickness and a universal interfacial law require further evidence.
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