  We compare U(1) lattice gauge theory to an effective model of Maxwell and
London equations. In the effective model there is only one free parameter, the
London penetration depth lambda. It turns out that one can get good agreement
between both models if one modifies the usual definition of magnetic monopole
currents in U(1) lattice gauge theory. This comparison also shows that already
at small distances fluctuations of the occuring string are important. Further,
we investigate the beta-dependence of the penetration depth and determine the
suppression of the monopole condensate in flux tubes.
