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\sect{FDT for RL-circuit, Nyquist theory}
Derive the Nyquist expression for the current-current correlation
function in a closed ring, taking into account its inductance.
Use the following procedure:
\begin{enumerate}
\item Cite an expression for the inductance $L$ of a torus shaped ring
given its radius~$R$ and its cross-section radius~$r$.
\item Write the R-L circuit equation for the current $I$,
where the flux $\Phi(t)$ through the ring is the driving parameter.
\item Identify the generalized susceptibility $\chi(\omega)$,
and observe that it is formally the same expression as in the problem of Brownian motion.
\item Calculate the current-current correlation function $\langle I(t)I(0)\rangle$,
taking the classical / high temperature limit.
\item Verify that $\langle I^2 \rangle $ agree with the canonical result.
\end{enumerate}