  We present first results for Faraday rotation of compact polarized sources (1
to 2 GHz continuum) in The HI/OH/Recombination line (THOR) survey of the inner
Galaxy. In the Galactic longitude range 39 degr < l < 52 degr, we find rotation
measures in the range -310 rad/m2 < RM < +4219 rad/m2, with the highest values
concentrated within a degree of l = 48 degrees at the Sagittarius arm tangent.
Most of the high RMs arise in diffuse plasma, along lines of sight that do not
intersect HII regions. For l > 49 degr, RM drops off rapidly, while at l < 47
degr, the mean RM is higher with a larger standard deviation than at l > 49
degr. We attribute the RM structure to the compressed diffuse Warm Ionized
Medium in the spiral arm, upstream of the major star formation regions. The
Sagittarius arm acts as a significant Faraday screen inside the Galaxy. This
has implications for models of the Galactic magnetic field and the expected
amount of Faraday rotation of Fast Radio Bursts from their host galaxies. We
emphasize the importance of sensitivity to high Faraday depth in future
polarization surveys.
