  Motivated by recent experiments on DNA torsion-force-extension
characteristics we consider the writhing geometry of open stiff molecules. We
exhibit a cyclic motion which allows arbitrarily large twisting of the end of a
molecule via an activated process. This process is suppressed for forces larger
than femto-Newtons which allows us to show that experiments are sensitive to a
generalization of the Calugareanu-White formula for the writhe. Using numerical
methods we compare this formulation of the writhe with recent analytic
calculations.
