The Magnetic Stirrer

A magnetic stirrer is a piece of laboratory equipment that uses a rotating magnetic field to cause a stirrer bar (also called "flea") immersed in a liquid to spin very quickly, thus stirring it.  The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath the vessel with the liquid.

Glass does not affect a magnetic field and most chemical reactions take place in glass vessels (i.e. beakers or flasks) and magnetic stirrer bars work well in glass vessels. However, the limited size of the bar means that magnetic stirrers can only be used for relatively small (under 4 litres) experiments.  They also have difficulty dealing with viscous liquids or thick suspensions.  For larger volumes or more viscous liquids, some sort of mechanical stirring is typically needed.

Magnetic Stirrer

 

Magnetic stirrers 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).  Due to its small size, a stirrer bar is more easily cleaned and sterilised 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 heating element to heat the liquid being stirred.

Arthur Rosinger of Newark, New Jersey, U.S.A. obtained US Patent 2,350,534, titled Magnetic Stirrer on 6 June 1944, having filed an application on 5 October 1942.   His patent includes a description of a coated bar magnet placed in a vessel, which is driven by a rotating magnet in a base below the vessel. His patent explains that coating the magnet in plastic or covering it with glass or porcelain makes it chemically inert.

The plastic-coated bar magnet was independently invented in the late 1940s by Edward McLaughlin, of the Torpedo Experimental Establishment (TEE), Greenock, Scotland, who named it the 'flea' because of the way it jumps about if the rotating magnet is driven too fast.

An even earlier patent for a magnetic mixer is US 1,242,493, issued 9 October 1917 to Richard H. Stringham of Bountiful, Utah, U.S.A. Mr. Stringman's mixer used stationary electromagnets in the base, rather than a rotating permanent magnet, to rotate the stirrer.

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