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We have studied the magnetic structure of AR7420 observed on 9 February 1993 by the Soft X-Ray Telescope (SXT) on Yohkoh. The observed X-ray images of the active region are compared with various force-free magnetic fields, extrapolated from the photospheric magnetograms obtained at the same time by the Huairou Station of the Beijing Astronomical Observatory. The results show that the magnetic configuration of the active region is not a potential field. Different groups of loops are characterized by different values of $\alpha$, indicating that the loops are twisted and that there are currents flowing through them. Here $\alpha$ is given by $\nabla\times B=\alpha B$ for a force free field, where B is the magnetic field. In particular, we find that there are opposite current systems in the active region, as specified by positive and negative values of $\alpha$. In addition, we find that there is no obvious correlation between the brightness of individual loops with the amount of twist. Implications of the results will be discussed.
