Why is it necessary to change the direction of the current on the webbing every half a turn?

The frame in a magnetic field tends to take a stable position when its plane is perpendicular to the direction of the magnetic induction of the external magnetic field. In order to disturb the balance of the frame and make it rotate in one direction, it is necessary to change the direction of either the magnetic field or the current in the frame. It is easier to change the direction of the current.

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