Magnetic stirrer

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Magnetic stirrer
A stir bar stirring a solution on a magnetic hot plate stirrer. The left knob controls the stirring rate, the right knob controls heating.
Other Names	Magnetic mixer
Uses	Liquid mixing
Inventor	Arthur Rosinger
Related	Stir bar Vortex mixer Static mixer

A magnetic stirrer is a type of laboratory equipment consisting of a rotating magnet or stationary electomagnets creating a rotating magnetic field. The stirrer is used to cause a stir bar, immersed in a liquid to be stirred, to spin very quickly, stirring it. Often, the stirrer can provide heating.

Stirrers are often used in laboratories, especially in the field of chemistry. They 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.

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.