= Kamerlingh Onnes's 1908 helium liquefaction experiment
{c}
On 10 July 1908, <Heike Kamerlingh Onnes>'s Leiden laboratory circulated purified <helium> through <liquid air> and evaporatively cooled liquid-<hydrogen> precooling, a regenerative <heat exchanger>, and an expansion valve. Returning <helium> cooled incoming <helium>; <liquid> collected in a transparent vacuum vessel. Reflected light revealed its <liquid meniscus>, crossed by <thermocouple> wires. Subsequent <mass density> and <spectroscopy> checks tested contamination.<The Liquefaction of Helium by Heike Kamerlingh Onnes>[Communication 108, §§2, 4–5, pp. 175–183]{ref}
\Image[/helium-liquefaction-refrigeration-chain.png]
{title=Functional stages of <helium liquefaction>}
{id=helium-liquefaction-refrigeration-chain}
{description=Original functional diagram of the reported sequence. Arrows show stream direction and <heat transfer>; positions do not represent the apparatus geometry. Compression and <heat> rejection close the refrigeration cycle.}
{source=helium-liquefaction-refrigeration-chain.py}
A constant-<volume (thermodynamics)> <helium> <gas thermometer> below the valve supplied the <temperature> indication. An <oxygen> <boiling> reference check disagreed with its expected reading; the <real gas> correction to <thermodynamic temperature> remained provisional. A <temperature> plateau suggested immersion before the <liquid meniscus> was recognized.<The Liquefaction of Helium by Heike Kamerlingh Onnes>[§4, pp. 180–181; §§5–6, pp. 183–184]{ref}
The measurement principles separate two questions. A visible <liquid meniscus> establishes an interface; a <gas thermometer> estimates <temperature> through an <equation of state>. A stationary <temperature> trace can also result from a <steady state> without <phase coexistence>, so it cannot replace the interface observation. Conversely, seeing a <liquid> does not calibrate a <thermometer>. <Thermometer calibration>, the <gas thermometer dead-space correction>, departures from the <ideal gas> law and incomplete <thermal equilibrium> all matter when assigning an absolute <temperature>. A check far from the intended measurement range tests the instrument but does not by itself bound every low-<temperature> error.
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