Why does my USB cable have an odd bulge towards one end of it?

Q I’ve noticed that cables supplied with USB devices often have a cylinder close to one end. Can you tell me what these are for? I have one such cable that was supplied with a scanner, which is now defunct. Is it okay to re-use the cable with other devices, or is it better to recycle it?
Ken Grainger

A These cylinders are ferrite beads, or chokes, which are designed to reduce electromagnetic interference – there is nothing more to them than that. From the look of the one in the photograph you sent us, it appears to be a clip-on variety: these may be used by manufacturers as a cheap way to modify an existing stock of cables.

You can even buy new ones for use on cables that lack such beads (if you have devices that are suffering from electromagnetic interference): Maplin, for instance, sells a pack of four clips for just over £2.

The answer to your question, then, is that it is perfectly safe to re-use the cable – with or without the ferrite choke. If you find it gets in the way, just unclip and remove it (though if you later experience problems with the device to which it’s attached, you will need to refit it).