  Light bridge (LB) is bright structure crossing the umbra of sunspots and
associated to the breakup or assembly of sunspots. In this paper, a LB is
presented and studied using the observatory data obtained by {\it Hinode}
satellites. Force-free factor ($\alpha$) and the z-component of current
($J_{z}$) and tension force ($T_{z}$) are calculated basing on the vector
magnetograms observed by Spectro-Polarimeter (SP) of the Solar Optical
Telescope (SOT) on board {\it Hinode}. It is found that the amplitudes of
$\alpha$ and $J_{z}$ of LB are generally larger than those of umbra. It is
found that there are two signs of $J_{z}$ along LB, which are divided at near
the middle position of LB. It is found that the amplitudes of $T_{z}$ of LB are
smaller than those of umbra and there are changes of sign of $T_{z}$ between
the boundary of LB and umbra. Through comparisons and investigations, it
suggest that LB and umbra maybe two different magnetic systems, which is a
necessary condition for interaction magnetic reconnection.
