Magnetic stirrer
From Free net encyclopedia
A magnetic stirrer consists of a small bar magnet (or stir bar), which is normally wrapped in plastics like PTFE 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 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. Coating the magnet in plastic or sealing it in a glass tube makes it chemically inert.
During operation, 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.
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 or water 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.