  A method for interpreting discontinuities of the twist potential of vacuum
stationary axisymmetric solutions of Einstein's equations is introduced.
Surface densities for the angular momentum of the source can be constructed
after solving a linear partial differential equation with boundary conditions
at infinity. This formalism is applied to the Kerr metric, obtaining a
regularized version of the density calculated with other formalisms. The main
result is that the integral defining the total angular momentum is finite for
the Kerr metric.
