  We derive the central charge and BPS equations from the low-energy effective
action for N=2 SU(2) Yang-Mills theory in the Coulomb phase, using a
systematic, canonical procedure. We then obtain solutions for monopole and dyon
BPS states, whose core structure is described by a dual Lagrangian containing
the monopole or dyon as a fundamental field. Spherically symmetric states
possess a shell of charge at a characteristic radius.
