to be used on the LaTeX wikibook. I created with the following LaTeX source:

\documentclass[a4paper,12pt]{article}

% New definition of square root:
% it renames \sqrt as \oldsqrt
\let\oldsqrt\sqrt
% it defines the new \sqrt in terms of the old one
\def\sqrt{\mathpalette\DHLhksqrt}
\def\DHLhksqrt#1#2{\setbox0=\hbox{$#1\oldsqrt{#2\,}$}\dimen0=\ht0
\advance\dimen0-0.2\ht0
\setbox2=\hbox{\vrule height\ht0 depth -\dimen0}%
{\box0\lower0.4pt\box2}}

\begin{document}

\begin{displaymath}
\sqrt{ \frac{a}{b} } \quad \oldsqrt{ \frac{a}{b} }
\end{displaymath}

\end{document}