Fleming's left-hand rule gives the direction of the force on a wire that carries a current at right angles to a magnetic field. Hold the thumb, the first finger and the second finger of your left hand at right angles to one another.
The first finger points in the direction of the magnetic field, from north to south. The second finger points in the direction of the current, from positive to negative. The thumb then points in the direction of the force, which is the way the wire moves. A simple memory aid is thuMb = Motion, First finger = Field, seCond finger = Current. It must be the left hand, because the right hand gives the wrong direction.
The rule shows how the three directions are linked. If the current is reversed, the second finger points the other way and the thumb swings round to point the opposite way, so the force reverses. If the current and the field are both reversed, the force points in the same direction as before.
