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The good ones aren’t cheap. They provide a ramped magnetic field which means you just press a button and that’s about it, although you would change the orientation once and repeat usually.
With a cheaper Chinese one like I have, you press the button and it’s on, and release and it’s off. This is not good for demagnetising as the sudden change in magnetic impulse can leave the item magnetised. So, you hold it on and slowly draw the item (watch) away from the source of alternating magnetism before switching off.
Is there a forum approved demagnetiser? It really seems like all my watches are - according to lepsi. I don’t have the money to send cheap watches for service.