\documentclass{article}
\usepackage[european, smartlabels]{circuitikz}
\begin{document}
\begin{circuitikz}[]
            \draw (0,2) to [open, o-o, voltage = straight] ++(0,-2)
            to ++(-4,0)
            to [vsource, v<=$u_\mathrm{in}(t)$, i=$i_\mathrm{in}(t)$] ++(0,2)
            to [variable european resistor] ++(4,0) ;
            \draw (0,2) to [open, v = $ u_\mathrm{in}(t)$, voltage = straight] (0,0);
 \end{circuitikz}
\end{document}