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The JCMT on Mauna Kea was used in April 1993 for polarization measurements of the continuum emission at a wavelength of 800 microns in the elongated cloud M17-SW. Magnetic fields in molecular clouds align the spinning dust grains so that the continuum dust emission is polarized with its electric vector perpendicular to the direction of the magnetic field. The extreme-infrared (submm) dust emission originates from the cores in molecular clouds, and it is not affected by scattering or absorption, so its polarization yields important information about the magnetic field direction inside star-forming regions. Here the direction of the magnetic field varies along the density ridge of the M17-SW cloud, so as to be roughly perpendicular to the local tangent to the cloud elongation.
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