Hot plate - Definition and Overview

A magnetic stirrer consists of a small bar magnet, which is normally wrapped in plastics like Teflon and a stand or plate containing an electromagnet. Often, the plate can also be heated. The bar magnet is placed in a vessel containing a liquid to be stirred. The vessel is set on top of the stand, where a rapidly rotating magnetic field causes the bar magnet to spin at high speed.

Magnetic stirrers are sometimes preferred to 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.

IKA - magnetic stirrer manufacturer (http://www.ika.net)