\begin{table} %t1 \caption{\label{t:dynamo}Parameters of the single ($m=0$) double periodic ($m=1$) cosine functions and their superposition ($m=0$/1) fitted to the rotation measure variations in the two rings of NGC~4254 (cf. Fig.~\ref{f:rmsect}). Amplitude, phase shift, background offset, and the Theil~U coefficient of the quality of fit are given. } \small%\centerline { \begin{tabular}{lrrrr} \hline \hline\noalign{\smallskip} Dynamo mode & Ampl. & Ph. shift & Offset & Theil U$^a$ \\ \hline RING 3\\ $m=0$ & 79 & 25 & 23 & 0.58 \\ $m=1$ & 53 & 43 & 41 & 0.83 \\ $m=0$/1 & 79/51 & 34 & 33 & 0.34 \\ \hline RING 4\\ $m=0$ & 82 & 21 & 2 & 0.57 \\ $m=1$ & 48 & 48 & 17 & 0.94 \\ $m=0$/1 & 71/36 & 33 & 9 & 0.36 \\ \hline \\ \end{tabular}} \smallskip $^a$~The Theil~U coefficient is limited to a range from zero to one (Theil \cite{theil72}). Zero means a perfect fit. \label{t:modes} \end{table}