I forget, but I think it's wound with #22 enameled magnet wire.  I got the wire from an electrician-type store in Midland, MI.  I walked in and told the guy what I was up to, and he was very interested in helping me out.  He gave me a spool of the wire to take home.  I wound the coil on bare 4.25" PVC in a few hours while watching TV.  My arms were dead when it was finally done.  More than once I slipped and had a real mess to re-wind.  After I finished I sprayed on several coats of polyurethane.  I took him the spool back and he charged me something like $4.  Woo hoo!