Question

What exactly is fleming's left hand rule in thermionic emissions

Answer 1

Fleming's  left hand rule principle is used in dc motor where the three fingers are perpendicular to each other where the thumb indicates direction of  force ,index finger indicates induced current & middle finger indicates the magnetic induction

Answer 2

If we stretch the first finger,the central finger and the thumb of left hand mutually perpendicular to each other such that the first finger points to the direction of magnetic field,central finger points the direction of electric current( motion of posiive charge) then the thumb represent direction of force experienced by the charge particle.