We have obtained new rotation measures for 11 pulsars observed with the Effelsberg 100-m radio telescope , in the direction of the Perseus arm .
Using a combination of 34 published and the 11 newly measured pulsar rotation measures we study the magnetic field structure towards the Perseus arm .
We find that two pulsars towards l \sim 149 ^ { \circ } ( Region 1 ) and four pulsars towards l \sim 113 ^ { \circ } ( Region 2 ) lie behind HII regions which seriously affect the pulsar rotation measures .
The rotation measure of PSR J2337+6151 seem to be affected by its passage through the supernova remnant G114.3+0.3 .
For Region 1 , we are able to constrain the random component of the magnetic field to 5.7 \mu G. For the large-scale component of the Galactic magnetic field we determine a field strength of 1.7 \pm 1.0 \mu G. This average field is constant on Galactic scales lying within the Galactic longitude range of 85 ^ { \circ } < l < 240 ^ { \circ } and we find no evidence for large scale field reversal upto 5-6 kpc .
We highlight the great importance to include the effects of foreground emission in any systematic study .