In this paper, an error estimator is used to control the quality of finite element solution in
linear magnetostatic. This method, based on the works of P. Ladevèze, consists in constructing a
complementary admissible field from the one calculated by the finite element method. The quality of
finite element solution is measured by comparing the couple of admissible solutions. This method
has been developed in 2D in the case of scalar and vector potential formulations. To show the
effectiveness of this error estimator, we present three simple examples, one of them having a known
exact solution.