[Noisebridge-discuss] thinksafe - a magnetic power connector for thinkpads

[Joseph Brenner]

On the same site:

  http://www.instructables.com/id/Fixing-a-Laptop-Charging-Cable/

There's a photo there of a repaired laptop power cable (lying on a
table with some rather stylish soldering iron burns).

My question is what is the cylindrical widget to the left of the
repaired connection?  These things are ubiquitous, and I've been
wondering what they are.  I cut one open once and it appeared to be a
dark ceramic cylinder with the cord running through it without any
direct electrical connection.  It's some sort of surge protection, I
would guess, but I have no idea how it works.