What is a solenoid with a ferromagnetic core?

electromagnet

Solenoids operate by means of electro-magnetic force when a current is passed through them. Iron is easily magnetised and is the only choice for the core.

It strengthens the magnetic field

inner core is solid outer core is liquid

All shipping Core i7 processors have four cores but they each run two threads at a time so in a sense it's 8. Your computer will even recognize it as 8 cores.