By the way you describe it, it sounds like a ferrite bead.  It's used for 
RFI suppression. (radio noise suppression).  Computer signals with fast rise 
and fall times create lots of harmonics of the fundamental frequency.  Thus 
these signals can interfier with radio communications and TV receivers.

William


----- Original Message ----- 
From: "Betti Ann and Preston Smith" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, May 22, 2007 9:02 AM
Subject: [PCBUILD] Cable question


> On my laptop and External USB drive I notice a small black cylinder 
> encapsulating individual USB and cables before they enter the devices
>
> What are they used for?  Surge protection? Smoothing the signals?
>
> Enquiring minds need to know
>
> Thanks,
>
> Preston
>
>                         PCBUILD's List Owners:
>                      Bob Wright<[log in to unmask]>
>                       Drew Dunn<[log in to unmask]>
> 

                  Visit our website regularly for FAQs,
               articles, how-to's, tech tips and much more
                          http://freepctech.com