  We report new imaging polarimetry observations of the Galactic compact HII
region K3-50 using CanariCam at the Gran Telescopio Canarias. We use a standard
polarimetric analysis technique, first outlined by Aitken, to decompose the
observed polarisation images centred at 8.7, 10.3, and 12.5 $\mu$m into the
emissive and absorptive components from silicate grains that are aligned with
the local magnetic field. These components reveal the spatially-resolved
magnetic field structures across the mid-infrared emission area of K3-50. We
examine these structures and show that they are consistent with previously
observed features and physical models of K3-50, such as the molecular torus and
the ionised outflow. We propose a 3D geometry for all the structures seen at
different wavelengths. We also compute relevant physical quantities in order to
estimate the associated magnetic field strengths that would be implied under
various physical assumptions. We compare these results with MHD simulations of
protostar formation that predict the magnetic field strength and configuration.
We find that the magnetic field may be dynamically important in the innermost
0.2 pc of the molecular torus, but that the torus is more likely to be
rotationally-supported against gravity outside this radius. Similarly, magnetic
fields are unlikely to dominate the {\em global} physics of the ionised
outflow, but they may be important in helping confine the flow near the cavity
wall in some locations. Ours is the first application of the Aitken technique
to spatially-resolved magnetic field structures in multiple layers along the
line of sight, effectively a method of "polarisation tomography."
