Magnetic field lines due to a conductor is as shown below. Then which of the following is true?
Current flows from A to B
According to right-hand thumb rule, imagine that you are holding a current carrying wire in your right hand so that the thumb points in the direction of the current, then the direction in which the fingers wrap the wire will represent the direction of magnetic lines of force.
In the given question, we are using the converse of this rule. If we wrap our right-hand fingers in the direction of magnetic field lines, then our thumb points in the direction of the current.
Applying the same rule in the given figure, we can say that current flows from A to B.