Possible Duplicate:
What is that cylinder on cables?
What's this part?
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closed as exact duplicate by Sathya♦, Joe Taylor, AndrejaKo, Arjan, Dennis Williamson Jan 17 '11 at 10:53
This question covers exactly the same ground as earlier questions on this topic; its answers may be merged with another identical question. See the FAQ for guidance on how to improve it.
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It is a Ferrite Bead. They're used to reduce and dissipate high frequency noise. To prevent it from moving on the cable, it is overmoulded with plastic. Functionally, it is a dissipative low-pass filter. It's basically an inductor with very high reluctance, so the magnetic field which it forms get dissipated as heat, rather than reflected back on the cable. | |||
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It could be a recording device disguised as a USB cable. | |||||
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The device in your picture is most likely a ferrite ring and is used to reduce RF interference. You'll also see them on a lot of video cables. You can also buy them as a separate item Have a look here | |||
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