Magnetic fields in spiral arms

In this paper two independent methods are described for estimating the magnetic field in the spiral arm in which we are located. The first method is based on an interpretation of the dispersion (of the order of 10° ) in the observed planes of polarization of the light of the distant stars; it leads to an estimate of H = 7.2 X 10 -6 gauss. The second method is based on the requirement of equilibrium of the spiral arm with respect to lateral expansion and contraction: it leads to an estimate of H = 6 X 10 -6 gauss.