Schematic illustrating “ON” and “OFF” RM positions, relative to an MC. To find the magnetic field in the MC we need to disentangle the RM produced by the Galaxy from that produced by the MC itself. We do so by subtracting the RM of a nearby point, called the OFF position, from the ON position which has an RM produced by both the MC (MC contribution) and the Galaxy (Galactic contribution). See Sect. 3.1 for details. Additionally, we need to consider the effects of all the layers of the MC from the exterior to the center of the cloud to reach to extinction value of A_{V, MC}= A_V −A_{V, ref}. However, since the cloud is symmetrical along the line-of-sight, and is illuminated from both sides byan ambient UV field, we assume the center of the cloud has an extinction of magnitudes.