  The magnetic scalar potential can be used to design electromagnets accurately
and efficiently. I will describe a practical construction algorithm: the
prescribed field in a "Target" region (constrained only by Maxwell's equations)
specifies boundary conditions which uniquely determine the potential in the
surrounding field "Return" region. As required by the physical representation
of the magnetic scalar potential, the conducting elements of the resulting coil
are directed along equipotentials on the surface of each region, at equal
increments of the potential. I give some examples and comments on the limits of
precision of the constructed field.
