Re: Laptop power supply question
It's a ferrite "bead" (or "beads") which attenuates RFI (Radio Frequency Interference) which could otherwise get back into your mains socket and cause interference to other devices plugged into the same mains supply. Unless you are using a radio on S/W or Medium Wave (AM) in the near vicinity, it generally doesn't make too much difference as to whether it's there or not - it's primarily there to satisfy some bureaucrat in the EU or he FCC.
OK?