What do the ferrite cores actually do?
D3100 learning curve:
The Quick Start pamphlet says that if you're going to use a third-party USB cable, you need to attach the provided ferrite core to it. It refers you to page 116 in the manual.
Go to the manual, first thing you'll notice is that it is smaller than before. That's because they have split it into a User Manual and Reference Manual. The latter is a PDF file on the provided CD. It's also available on the web. On page 116 it says the same thing, attach the smaller of the provided ferrite cores to your third-party USB cable. The diagram indicates that the ferrite cores snap together to enclose the cable near the end that you plug into your camera.
I was hoping to use a USB cable from my Panasonic DMC-L10, which appears identical to the USB cable supplied with my D5000, including an integral ferrite core. I already know that the L10's cable will work with the D5000 and vice versa. But I am wondering now, what exactly does the ferrite core do, and why do "third-party" USB cables need to have one added?
You can always buy Nikon's UC-E4 USB cable separately. I can see what they're doing to cut costs but this seems a little unwise.