  The current-carrying ability of a type-II superconductor is generally
represented by its critical current density. This can be determined by
measuring a flux relaxation process starting with a testing current density
that is greater than or equal to the critical value. Here we show that a flux
relaxation process starting with an intermediate current density can be
converted into a process starting with the critical current density by
introducing a virtual time interval. Therefore, one may calculate the critical
current density from the flux relaxation process starting with a current
density below the critical value. The exact solutions of the time dependence of
current density in the flux relaxation process were also discussed.
