See the short introductory presentation to this topic.
Image at right: A strong current points into the page through the hole, causing reversal
of the circulation of the vortex pinned on the hole, by the production of both a positive and
negative vortex that exit the system! The initial circulation was counterclockwise. The currrent
forces clockwsie circulation to be lower energy. The spin configuration was produced by a
Monte Carlo simulation. ------>