Magnetic stirrer

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A stir bar stirring a solution on a magnetic hot plate stirrer. The left knob controls the stirring rate, the right knob controls heating.

A magnetic stirrer consists of a small bar magnet (or stir bar), which is normally coated in a plastic such as PTFE and a stand or plate containing a rotating magnet or stationary electomagnets creating a rotating magnetic field. Often, the plate can also be heated. During operation of a typical magnetic stirrer, the bar magnet (or flea) is placed in a vessel containing a liquid to be stirred. The vessel is set on top of the stand, where the rapidly rotating magnetic field causes the bar magnet to rotate.

Arthur Rosinger of Newark, New Jersey, U.S.A. obtained US Patent 2,350,534, titled Magnetic Stirrer on 06 June 1944, having filed an application therefor on 05 October 1942. Mr. Rosinger's 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. Mr. Rosinger also explains in his patent 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 09 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.

The first multipoint magnetic stirrer was developed and patented by Salvador Bonet of SBS Company in 1977. He also introduced the practice of noting the denomination of stirring power in "liters of water", which is a market standard today.

Magnetic stirrers are preferred over gear-driven motorized stirrers in chemical research because they are quieter, more efficient, and have no moving parts to break or wear out (other than the simple bar magnet itself). Due to the small size, the stirring bar is more easily cleaned and sterilized than other stirring devices.

Magnetic stirrers solve two major problems with using motorized stirrers. Firstly, motorized stirrers use lubricants, which can contaminate the vessel and the product. Secondly, in motorized stirrers, the sealing of the connection between the rotating shaft of the stirrer and the vessel can be a problem, especially if a closed system is needed (e.g. due to OSHA, or environmental regulations, or because a process works only if oxygen, water or dust is absent).

The magnetic stirrer also has its drawbacks: the limited size of the stirring bar means it can only be used for lab size experiments. In addition, viscous liquids or thick suspensions are extremely difficult to stir using this method, although there are some models with special magnets to overcome this problem.