Magnetic stirrer
Encyclopedia
A magnetic stirrer or magnetic mixer is a laboratory device that employs a rotating magnetic field to cause a stir 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. Magnetic stirrers often include a hot plate
or some other means for heating the liquid.
Magnetic stirrers are often used in chemistry
and biology
. They 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 stirring bar is more easily cleaned and sterilized than other stirring devices. They do not require lubricant
s 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.
On the other hand, the limited size of the bar means that magnetic stirrers can only be used for relatively small (under 4 liters) experiments. They also have difficulty dealing with viscous
liquids or thick suspensions.
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.
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.
Heating elements may range from 120 W
or lower to 500 W or more. The maximum reachable fluid temperature depends on the size of the flask, the quantity of solution to be heated, and the power of the heating element.
Stir bar
A stir bar is a magnetic bar used to stir a liquid mixture or solution, usually in a laboratory. The stir bar's motion is driven by a separate rotating magnet or assembly of electromagnets located beneath the vessel containing the liquid...
(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
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
or a set of stationary electromagnets, placed beneath the vessel with the liquid. Magnetic stirrers often include a hot plate
Hot plate
A hot plate is a portable self-contained tabletop small appliance that features one, two or more gas burners or electric heating elements. A hot plate can be used as a stand alone appliance, but is often used as a substitute for one of the burners from an oven range or the cook top of a stove...
or some other means for heating the liquid.
Magnetic stirrers are often used in chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
and biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
. They 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 stirring bar is more easily cleaned and sterilized than other stirring devices. They do not require lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
s 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.
On the other hand, the limited size of the bar means that magnetic stirrers can only be used for relatively small (under 4 liters) experiments. They also have difficulty dealing with viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
liquids or thick suspensions.
History
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 therefor on 5 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
Greenock
Greenock is a town and administrative centre in the Inverclyde council area in United Kingdom, and a former burgh within the historic county of Renfrewshire, located in the west central Lowlands of Scotland...
, Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, 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.
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.
Heating elements may range from 120 W
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
or lower to 500 W or more. The maximum reachable fluid temperature depends on the size of the flask, the quantity of solution to be heated, and the power of the heating element.
External links
- DIY Stir plate Make your own stir plate from an old computer fan.
- Short video of a home made stir plate.
- http://www.google.com/patents/about?id=3CxTAAAAEBAJ&dq=US+Patent+2,350,534 Arthur Rosinger's patent on "Magnetic Stirrer"