  We present 850 micron imaging polarimetry of the W51A massive star forming
region performed with SCUBA on the JCMT. From the polarimetry we infer the
column-averaged magnetic field direction, projected onto the plane of the sky.
We find that the magnetic field geometry in the region is complicated. We
compare the field geometry with 6 cm and CS J=7-6 emission and determine that
the magnetic field must be relatively weak and plays a passive role, allowing
itself to be shaped by pressure forces and dynamics in the ionised and neutral
gases. Comparisons are drawn between our data and 1.3 mm BIMA interferometric
polarimetry data, from which we conclude that the magnetic field must increase
in importance as we move to smaller scales and closer to sites of active star
formation.
