  We reexamine the Kosterlitz-Thouless phase transition in the ground state
$|\Psi_0\rangle$ of an antiferromagnetic spin-$\frac{1}{2}$ Heisenberg chain
with nearest and next-nearest-neighbor interactions $\lambda$ from a different
perspective: After defining winding number (topological charge) $W$ in the
basis of resonating valence bond states, the finite-size scaling of
$\langle\Psi_0|W| \Psi_0 \rangle$, $\langle\Psi_0|W|\partial_\lambda \Psi_0
\rangle$, $\langle\partial_\lambda\Psi_0|W|\partial_\lambda \Psi_0 \rangle$
leads to the accurate value of critical coupling $\lambda_c=0.2412\pm0.0007$
and to the value of subleading critical exponent $\nu=2.000\pm0.001$. This
approach should be useful when examining the topological phase transitions in
all systems described in the basis of resonating valence bonds.
