Magnetic stirrer
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A magnetic stirrer consists of a small bar magnet, which is normally wrapped in plastics like Teflon and a stand or plate containing a rotating magnet. Often, the plate can also be heated. The plastic-coated bar magnet was invented in the late 1940's 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. Coating the magnet in plastic (it can also be sealed in a glass tube) makes it chemically inert.
The bar magnet (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 spin at high speed.
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 any other stirring device.
There are two major problems using motorized stirrers. One are lubricants, which can contaminate the vessel and the product of course. The second problem is the sealing of the connections between the rotating shaft of the stirrer and the vessel, if a closed system is needed (e.g. OSHA or environmental regulations, process works only if oxygen or water dust is absent, you need a sterilized system).
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.
External Links
IKA Laboratory Equipment (magnetic stirrer manufacturer) (http://www.ika.net)