Maxwell’s right hand thumb rule. According to this law if we hold our thumb of right hand in the direction of magnetic field and close our finger then our enclosed finger shows the magnetic field line around the conductor.

Let AB be a straight wire. Placed vertically A to B. To find the direction of magnetic field we assume that we are holding the current carrying wire in such a way that our thumb of right hand shows the direction of flow of current. Then the finger enclosing the wire. The direction in which our finger is enclosed shows the direction of magnetic field which is anticlockwise.