<P> A stir bar is the magnetic bar placed within the liquid which provides the stirring action . The stir bar's motion is driven by another rotating magnet or assembly of electromagnets in the stirrer device, beneath the vessel containing the liquid . Stir bars are typically coated in PTFE, or, less often, in glass; the coatings are intended to be chemically inert, not contaminating or reacting with the reaction mixture they are in . Glass coatings are used for liquid alkali metals (except lye, which will eat through glass) and alkali metal solutions in ammonia . </P> <P> They are bar shaped and often octagonal in cross-section (sometimes circular), although a variety of special shapes exist for more efficient stirring . Most stir bars have a pivot ring around the center on which they rotate . The smallest are only a few millimeters long and the largest a few centimeters . A stir bar retriever is a separate magnet on the end of a long stick (usually coated with PTFE) which can be used to remove stir bars from a vessel . </P>

Why should you not stir the solution with a thermometer