What is the name of the oblong black shell thing that is (presumably) connecting the two halves of the cable? There is a small one and a large one shown in the image below.
I've searched around for electrical cable connectors/joiners, but haven't been able to find this exact part. They seem pretty common, I have seen a few on USB cables I have.
I'd like to buy some of these for a project, so if you know what to search for on eBay to find these, I'd appreciate it.
EDIT: Thank you for the answers. I was actually looking for some sort of in-line compartment to hold a little circuit, and was surprised to learn about ferrite cores.