12pcs Inner Diameter 9mm 0.35 inch Noise Filter Ferrite Core Ferrite Clamps Ferrite Clip Ferrite Chokes Ferrite Snap Ferrite Bead 2035-0930 Black

Great

These are what you need to reduce RF interference from switching power supplies. (wall warts)

I measured my power cord at 9mm with a ruler, since the next size up is 13mm I didn't think that the tolerance would be critical. The items purchased are for a 9mm power cord. The tolerance is too tight and with filter will not close without having to apply so much pressure that the ferrite cracks. Only use this if your power cord can be measured with a micrometer to be less than 9mm. EDIT: I ended up taping two of the ferrite filters to the microwave power cord. It did the trick! EDIT2: This morning, the problem returned. Not sure why.

I ordered these clip on ferrite filters for use on my milling machine to cut down on electro-magnetic interference (EMI). While 9 of the 10 items worked fine, one of them wouldn't open...no, not the outer plastic shell, but the magnet within. It's like the magnet is glued shut... I couldn't pry it open or anything. Not good if you need to attach it to an existing cord with a plug end.

Used on a radio to cut interference from LED lights and they don't work. Didn't cut the interference even slightly. Absolutely useless. That's why I'm returning them.