= Platinum resistance thermometer
A <resistance thermometer> using <platinum> as its sensing material. It provides the <temperature> readout in the <saturation-pressure equilibrium measurement>.
Transduction turns <electrical resistance> into a <temperature> indication. A uniform wire has $R=\rho_{\rm el}\ell/A$, with <electrical resistivity> $\rho_{\rm el}$, <length> $\ell$, and cross-sectional <area> $A$. Stronger atomic vibrations at higher <temperature> scatter <Electrons> more, increasing <platinum>'s <electrical resistivity>; impurities and <crystallographic defects> also contribute.<Guide on Secondary Thermometry: Industrial Platinum Resistance Thermometers by Jonathan Pearce and others>[Guide on Secondary Thermometry: Industrial Platinum Resistance Thermometers, §2.1, printed pp. 6–7: resistivity, electron scattering, and impurities]{ref}
A small known <electric current> $I$ and measured <electric potential difference> $V$ give $R=V/I$ through <Ohm's law>; a <resistance bridge> can instead compare the element with a reference <resistor>. Inverting the calibrated response then gives the sensing element's <temperature>. This makes electrical readout convenient, but the <platinum resistance thermometer lead resistance error>, <resistance thermometer self-heating>, and <resistance thermometer immersion test> explain why a stable display alone is insufficient. The <gas thermometer> supplies a contrasting route through an <equation of state>; the <saturation-pressure equilibrium measurement> shows why a trustworthy independent <temperature> readout matters.
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