= Radiometer thermal link
{title2=$Q_{\rm link}\simeq G(T-T_b),\ \tau=C/G$}
A <radiometer thermal link> is the intentional path for <thermal conduction> from receiver to reference block. Near a chosen operating point, write $Q_{\rm link}\simeq G(T-T_b)$ with local conductance $G$ in $\mathrm{W\,K^{-1}}$. A weaker link increases the steady <temperature> rise per added <power>, $\delta T\simeq\delta P/G$, but also increases the uncontrolled relaxation <time> $\tau=C/G$ when the link dominates other <heat transfer>. These are local linear approximations: actual <thermal conductivity> depends on <temperature>, and active <feedback control> introduces its own response. NIST explicitly describes the tradeoff between weaker links and smaller receiver <heat capacity>.<Pico-Watt ACR by National Institute of Standards and Technology>[Key Design Features, final paragraph: response time proportional to receiver heat capacity and inverse thermal conductance]{ref}
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