[electroking]

Originally Posted by old_coot88

To expand a bit on Eric's excellent synopsis, think of winnowing wheat for an analogy. The wind separates the light stuff, the chaff, from the falling grain which is much heavier.
In the CRT with the crooked gun, the the entire stream is directed off-center so the heavy stuff (the ions) is dumped harmlessly aside, while the light stuff (the electrons) are deflected back down the center of the tube. The magnet is actually an 'electron beam re-directer'.

Ions, being charged atoms, actually are affected by magnets, but far less so than electrons which are much lighter and easier to deflect (per the analogy).
For all practical purposes here, the ions are not deflected enuff to matter.

I couldn't say it better...