A circle doesn't cancel a parabola

After I had used the parallel plates to bend the electron beam down in a parabola, I turned on the Helmholtz coils to produce a magnetic force upwards on the beam. To work out the direction of the magnetic field we use Fleming's Left Hand Rule. The middle finger means conventional current, a flow of positives. So for electrons I point my middle finger back to the source of electrons and then my thumb up to show the magnetic force upwards. My first finger points towards me so the north pole is on the back and the south pole is nearest to use. The electron beam should now go straight. It is bent because a magnetic field sends electrons along a circular path and a circle cannot fully cancel out a parabola.