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I haven't seen any for a while now, but a couples of year ago I saw some several times: cylindrical magnets around electrical wires. I'm not talking about specialized equipments but items of everyday life which comes with such a magnet. In fact I did not know it was a magnet until I accidentally broke one. Unfortunately I did not manage to find any picture from the web.

Do you know the use of them? Also, are we still using them?

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They're not actually magnets, but rather ferrite which is a paramagnetic material. A ferrite bead with a conductor through it is an inductor and so is used as a low pass filter. Typical use is for power cables to reduce EMI (electromagnetic interference).

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    \$\begingroup\$ I accepted this answer because of the second sentence. KyranF's answer was good too though... I had to choose. \$\endgroup\$ – anderstood Oct 12 '14 at 13:37
  • \$\begingroup\$ @anderstood haha, thanks anyway. The use of them is really for anything that can emit/absorb noise, both signal and power connections \$\endgroup\$ – KyranF Oct 12 '14 at 19:18
  • \$\begingroup\$ The trick is that when you put multiple wires through them they act as a "common mode choke". So high frequencies between different wires inside the cable can pass but high frequency common mode signals (which generally represent unwanted interference trying to get in or out of the system) are strongly attenuated. \$\endgroup\$ – Peter Green Jan 18 '16 at 16:18
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They are ferrite (magnetic) rings, over cables in order to suppress noise and filter out unwanted external interference. They are often placed over USB cables, I got one included in my Nikon camera box. Check out the Wiki article for more in depth descriptions and some pictures.

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