  Pulsars provide unique probes of the large-scale interstellar magnetic field
in the Galactic disk. Up to now, the limited Galactic distribution of the known
pulsar population has restricted these investigations to within a few
kiloparsec of the Sun. The Parkes multibeam pulsar survey has discovered many
more-distant pulsars which enables us for the first time to explore the
magnetic field in most of the nearby half of the Galactic disk. Here we report
the detection of counterclockwise magnetic fields in the Norma spiral arm using
pulsar rotation measures. The fields are coherent in direction over a linear
scale of $\sim 5$ kpc along the arm and have a strength of $-4.4\pm0.9$ $\mu$G.
The magnetic field between the Carina-Sagittarius and Crux-Scutum arms is
confirmed to be coherent from $l\sim45\degr$ to $l\sim305\degr$ over a length
of $\sim 10$ kpc. These results strengthen arguments for a bisymmetric spiral
model for the field configuration in the Galactic disk.
