  The dually transformed path integral of four-dimensional U(1) lattice gauge
theory is used for the calculation of expectation values in the presence of
external charges. Applying the dual simulation to flux tubes for charge
distances up to around 20 lattice spacings, we find a deviation from the
behaviour of a dual type-II superconductor in the London limit at large
distances. The roughening of the flux tube agrees with the effective string
picture of confinement. Further we show that finite temperature effects are
negligible for time extents larger than the charge distance. Finally we analyze
the different contributions to the total energy of the electromagnetic field.
The parallel component of the magnetic field turns out to have the strongest
influence on the deviation from the linear behaviour at small charge distances.
