electromagnet research

The best core to use is soft iron. If cast iron or steel is used, it will be magnetized after the current ceases.
Thinner wire is better because you can fit more turns in the same space.

Tip: mount the core in a drill.

You can vary the strength by varying the voltage.

Computer power supplies give +5V and +12V and they won't run out of battery in the middle of the performance.  They are free (trash).  But they have pretty low current.

Maybe use a spool of wire because it's already wrapped.

Professional electromagnets often have thin water coils built in for cooling.  Aluminum cooling fins may work too.

http://www.coolmagnetman.com/magdcem.htm