Since the positively charged particle (alpha-particle) projected towards west, so the direction of current is towards west. Now the deflection is towards north, so the force is towards north' Now hold the forefinger, centre finger and thumb of our left-hand at right angles to one another' Let us adjust the hand in such a way that our centre finger points towards west and thumb points towards north. If we look at our forefinger, it will be pointing, upward. Thus, the magnetic field is in the upward direction. So, the correct answer is (iv).
