Preston,

This sounds like a ferrite core or bead. These are used to suppress RF 
(Radio Frequency) signal interference that might be present on the wires. 
Without these beads, the signals would travel down the length of the wire 
and possibly emit radio interference along the path of the wire.

These ferrite beads are usually made of powdered iron that has been pressed 
together into the desired shape.

Russ Poffenberger
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At 06:02 AM 5/22/2007, you wrote:
>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

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