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Published online by Cambridge University Press: 14 August 2015
A circular arm with elliptical cross-section is used as a model of the spiral arm. It is demonstrated magnetohydrodynamically that interstellar gas may flow in a helical path along the axis of the arm and interstellar helical magnetic lines of force can spiral around it. It is pointed out that though the rolling motions of gas do not always indicate the existence of the helical magnetic field, the latter cannot be in a stationary state without the former. Some observational supports to the present model are given.