I'm making my first stir and don't have a ton of time to fiddle around. I see everyone's stir plates use two magnets, and I don't know how the polarity on the stir bar is arranged, but I assume at the ends. Therefore I assume you must use two magnets to get matching poles at the ends instead of at the top and bottom. Is this all correct? Has anyone used a single round magnet or bar magnet? Thanks!