  The chemical potential of a quantum dot parabolically confining one or two
electrons is studied in an axial magnetic field. The number of electrons in the
dot is given by combination between the chemical potentials of the dots and
leads. The equilibrium-charge diagram of the dot is obtained in the phase space
determined by the magnetic field and the lead voltage. The influence of the
magnetic field on the electron number and the electron transport is discussed.
The dependence of the electron number on the magnetic fields and the lead
voltage is clear at a glance in the equilibrium-charge diagram. Moreover, the
diagram is a compact tool to show the condition of the electron transport.
