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Possible Duplicate:
What is that cylinder on cables?

What's this part?

alt text

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marked as duplicate by Sathya, Joe Taylor, AndrejaKo, Arjan, Dennis Williamson Jan 17 '11 at 10:53

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

up vote 13 down vote accepted

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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Why is high frequency noise a problem on a laptop power cable? – jwg May 15 '14 at 9:26
@jwg - Because laptops are excellent sources of it! It's actually more to prevent the laptop radiating electromagnetic noise then receiving it. – Fake Name May 15 '14 at 9:48
Ok, that makes sense, thanks! – jwg May 15 '14 at 10:03

It could be a recording device disguised as a USB cable.

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+1 for urban myth potential :) – Sirex Jan 17 '11 at 10:19

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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