  Within a simple elastic theory, we study the elongation versus force
characteristics of a supercoiled DNA molecule at thermal equilibrium in the
regime of small supercoiling. The partition function is mapped to the path
integral representation for a quantum charged particle in the field of a
magnetic monopole with unquantized charge.
  We show that the theory is singular in the continuum limit and must be
regularised at an intermediate length scale. We find good agreement with
existing experimental data, and point out how to measure the twist rigidity
accurately.
