Equation 23.1.28A811248FC3F3

\begin{equation} \sigma =2aq,\qquad \sigma _ x=-\kappa /\eta ,\qquad \mathrm{d}x=-\frac{\mathrm{d}q}{a}=\sqrt{\frac\eta {2\kappa }}\frac{\mathrm{d}t}{tq}. \end{equation}