Count the marks inside the can and count the pleats of the filter. I bet you will find the same number. A magnet stuck on the bottom of the can would attract ferrous metal (steel and iron only) to the bottom of the can.
Totally wrong assumption. The lines of ferrous dust does NOT correspond to the pleats at all. You're 100% wrong on that count.
I don't care where you put the magnet(s), and I don't care if they are catching ferrous particles which may(or may not) be caught by the filter element. They are obviously doing something that I can visually confirm.
