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We utilized the Very Large Array to make multifrequency polarizationmeasurements of 20 radio sources viewed through the IC 1805 HII region and"Superbubble", as well as in the immediate vicinity. The measurements atfrequencies between 4.33 and 7.76 GHz yield Faraday rotation measures along 27lines of sight to these sources (some sources have more than one component).The Faraday rotation measures (RM) are used to probe the plasma structure ofthe IC 1805 HII region and to test the degree to which the Galactic magneticfield is heavily modified (amplified) by the dynamics of the HII region. Wefind that similar to the Rosette Nebula (Savage et al. 2013, Costa et al. 2016)and the Cygnus OB1 association (Whiting et al. 2009), IC 1805 constitutes a"Faraday rotation anomaly", or a region of increased RM relative to the generalGalactic background value. Although the RM observed on lines of sight throughthe region vary substantially, the |RM| due to the nebula is commonly 600 --800 rad m^-2. In spite of this, the observed RMs are not as large as simple,analytic models of magnetic field amplification in HII regions might indicate.This suggests that the Galactic field is not increased by a substantial factorwithin the ionized gas in an HII region. We also find that with one exception,the sign of the RM for all sources is that expected for the polarity of theGalactic field in this direction. The same behavior was found for the RosetteNebula, and qualitatively indicates that turbulent fluctuations in the Galacticfield on spatial scales of $\sim 10$ pc are smaller than the mean Galacticfield. Finally, our results show intriguing indications that some of thelargest values of |RM| occur for lines of sight that pass outside the fullyionized shell of the IC 1805 HII region, but pass through the PhotodissociationRegion (PDR) associated with IC 1805.

A Faraday Rotation Study of the Stellar Bubble and HII Region Associated with the W4 Complex