Under the usual electromagnetic conditions, magnetic flux lines and current are perpendicular to each other, and electromagnetic phenomena are determined by the balance between the Lorentz force and the pinning force. On the other hand, various peculiar phenomena are observed when a current is applied to a superconductor in a parallel magnetic field. These phenomena are introduced in this chapter. These phenomena are also determined by a general driving force on flux lines and the pinning force to maintain the distorted structure of the flux lines. The Lorentz force is zero, however, because the current and magnetic flux lines are parallel to each other. Hence, another force is driving the flux lines. Following the general idea that the driving force is a restoring force to release the distortion, it is easy to suppose that it is a torque to rotate the flux lines. In fact, this torque can be derived similarly from the principle of virtual displacement as was done in the derivation of the Lorentz force in Chap. 5 . Various peculiar phenomena associated with the longitudinal magnetic field effect are generally explained by the rotational flux motion caused by the torque. These phenomena can occur only in superconductors with the pinning effect, and new phenomena that have not been considered in electromagnetism can be seen.