Unofficial
FEMM
documentation
by Dr Stan Zurek

In FEMM:

If no boundary conditions are explicitly defined, each boundary defaults to a homogeneous Neumann boundary condition. However, a non-derivative boundary condition must be defined somewhere (or the potential must be defined at one reference point in the domain) so that the problem has a unique solution.
For axisymmetric magnetic problems, A = 0 is enforced on the line r = 0. In this case, a valid solution can be obtained without explicitly defining any boundary conditions, as long as part of the boundary of the problem lies along r = 0. This is not the case for electrostatic problems, however. For electrostatic problems, it is valid to have a solution with a non-zero potential along r = 0.

Symmetry of the geometry can be exploited by by using the Dirichlet or Neumann conditions:

• Dirichlet - the condition of A=0 is enforced along the boundary which stops the flux crossing that boundary. It can be thought of as a perfect insulator.
• Neumann - the condition of dA/dn = 0 which means that the flux lines will be forced to cross this boundary at 90 deg angle. It can be though of as a perfect conductor.