How a Magnet Stirring works

When you use a magnetic stirrer in the lab, you rarely think about how it does what it does. You might not even care why it works; instead, you may be happy knowing that it’s doing its job and stirring up your reaction mixture. However, when you find out how the magnet works, you’ll never look at one the same way again. This guide explores how a magnet stirring works and what makes it unique.

How a Magnet Stirring works

A magnetic stirrer works through eddy currents. Many people have probably heard of electromagnetic waves before-radio waves, microwaves, infrared waves, visible light are all electromagnetic waves. An electromagnetic wave refers to an oscillating electric field that stimulates the movement of charged particles in a certain direction. 

A changing magnetic field can be thought of as creating an electric current or moving charges. When these alternating currents come into contact, they create another type of wave called the eddy current, which causes periodic rotations around the conductor’s axis.

What makes this mixer different from any other?

The main difference between a regular magnetic stirrer and one with an electromagnetic design is the frequency of the magnetic field. The former uses a DC rotating magnetic field  (~500 Hz), while the latter applies an AC rotating field (~120 kHz). What does that mean? In essence, it means that eddy currents are generated about 120 thousand times per second in a typical electromagnetic stirrer, compared to about 500 times for a standard one. That means more energy is transferred from the magnet to your medium because so many more vibrations are happening.

Another interesting difference has to do with how the stir bar works in these two types of mixers. In a standard magnetic stirrer, what’s moving around is just the magnet itself (the metallic part). However, since this creates powerful eddy currents in whatever liquid you’re stirring, the stir bar is dragged along with it. In an electromagnetic mixer, however, it’s the eddy currents that move, and they push on the stir bar in a rotating motion in one direction and then another in a repeating cycle. These two mixers are often used for different purposes: A DC stirring mechanism can get intense mixing from a very strong magnet (although you need a large stir bar to accommodate this). At the same time, an AC one allows you to use much smaller magnets but get more intense energy transfer at higher frequencies.