Step 7: P.2, Build: Wind an Electromagnet, the Right Way
Start by liberating the core (Picture 2) holding the windings in an AC induction motor - almost any microwave oven will contain a fan driven by such a motor (Picture 1). Be careful with disassembly - Plasmana's instructable shows you how it's done.
When dremeling, always wear safety eyewear, and use a vice if you can (less chance of injuring or killing yourself).
Once the core containing the windings has been removed, we rip them off and replace them with our own. Note that in addition to wrecking the induction motor's frame, the dremel is also a good tool for ruining the windings.
The reason we use such a core is to avoid saturation, which is common in electromagnets that are wound around nails.
Pick any direction (CW or CCW, it doesn't matter) and start winding 28 gauge magnet wire from the side of the electromagnet that will be attached to the end of the arm. My core had a C on one side that made it convenient for zip typing to the end of the arm.
Once you've wound a layer, tape it over (Picture 3) and run the wire all the way back to the other side (Picture 4). Continue this process until no more layers fit on the core, or until you feel the magnet is strong enough. Be sure you always wind in the same direction! You can test the magnet's strength by sanding off a small section of wire (without cutting the wire) on the side you're winding, and then running some current through it. If you're using a high discharge Lipo battery, you may want to add a current-limiting resistor. With the core and number of windings I had, I found that my battery was able to source enough current without becoming sad.
Once the winding is complete, cut the magnet from the roll of enameled wire, strip the ends, solder them to long wires that will go down the arm back to the controller, and apply electrical tape or heatshrink to insulate the contacts. Finally, twist the wires together for durability and neatness (Picture 5).