The picture of the magnetic field of the Galaxy discussed in earlier papers is unsatisfactory in that it does not provide for an outflow of gas from the galactic nucleus. The observed lack of any inward flow of gas suggests that the interstellar medium must constantly gain angular momentum. But unless the galactic halo holds the main reservoir of gas and angular momentum, it is difficult to see how this can be the case. It is now suggested that, instead of the spiral arm field being primary and the halo field secondary, as it was in our earlier work, the situation may be reversed. In a model in which lines of force of the halo cross the interarm regions of the galactic plane, it is possible for the whole interstellar medium to gain angular momentum at a comparatively rapid rate. The windings of the spiral arms then move outwards, thereby preventing the arms from becoming more tightly wound with time. When the field intensity in the halo is 3 × 10 −6 gauss, the windings move out to the periphery of the Galaxy in a time scale of order 5 × 10 8 years. The structure of the spiral arm field, according to this new picture, is briefly discussed. It seems that a helical structure is to be preferred to the usual cylindrical model. A well-known difficulty concerning the direction of the local spiral arm is then overcome.