Imagine you’ve gone back in time, but you’re hankering for a few of the amenities you’ve grown accustomed to living in the 21st century. Hopefully you remembered to bring your Time Traveler’s Cheat Sheet, a handy guide to inventing some of life’s essentials.
A moving electric field produces magnetism, and vice versa, wrap copper wire around an iron core and run electricity through it, and you’ve got an electromagnet. Don’t have any electricity? Put a magnet on a water wheel, and put your copper-wrapped iron beside the wheel, and hey presto, you’re converting mechanical energy into electricity.