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Four magnetic stir bars with a meter stick (the bottom right stir bar is also in the other picture)

A stir bar (or flea) is a magnetic bar, used to stir a liquids in a laboratory. The stir bar rotates (and thus stirs) in synch with a separate rotating magnet located beneath the vessel containing the reaction.^[1] Glass does not affect a magnetic field appreciably (it is transparent to magnetism) and most chemical reactions take place in glass vessels (i.e. see beaker (glassware) or laboratory flasks). This allows magnetic stir bars to work well in glass vessels.

Stir bars are typically coated in teflon (or less often in glass), so that they are chemically inert and do not contaminate or react with the reaction mixture they are in.^[1]

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 ridge around the center (called a pivot ring) on which they rotate. The smallest are only a few millimeters long and the largest a few centimeters.

A stir bar stirring a solution on a hot plate stirrer

A stir bar retriever is a separate magnet on the end of a long stick (usually coated with teflon) which can be used to get (or fish) stir bars out of the reaction vessel.^[1]

The magnetic stirrer can be as simple as a magnet spun by flowing air or water, on which the reaction vessel containing the stir bar sits. Most magnetic stirrers today spin their magnets with an electric motor. If it also has a heating element, it is referred to as a 'hot plate stirrer'.

Stir bars work best for relatively small reactions (a few liters of volume at most) that are not very viscous. For larger volumes or more viscous liquids, some sort of mechanical stirring is typically needed.

1. ^ ^{a b c} Colorado University, Boulder: Organic Chemisty Undergraduate Courses. Equipment: Spin Bar (Stir Bar). Retrieved on December 11, 2006.