The rotating field may be created either by a
rotating magnet or a set of stationary electromagnets, placed beneath
the vessel with the liquid. Since glass does not affect
a magnetic field appreciably (it is transparent to magnetism), and
most chemical reactions take place in glass vessels, magnetic stir bars work
well in glass vessels.
Magnetic stirrers are often used in chemistry and biology.
They are preferred over gear-driven motorized
stirrers because they are quieter, more efficient, and have no moving
external parts to break or wear out (other than the simple bar magnet itself).
Because of its small size, a stirring bar is more easily cleaned
and sterilized than other stirring devices. They do not
require lubricants which could contaminate the reaction vessel and
the product. They can be used inside hermetically closed vessels or systems,
without the need for complicated rotary seals.
Magnetic stirrers may also include a hot plate or some
other means for heating the liquid.