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Magnetic tweezers consist in a set of magnets producing a strong field gradient which is used to exert a force on magnetic beads. The force is varied by modulating the position of the magnets relative to the the bead. Usually, magnets are millimeters in size whereas the bead is only allowed to move by a few microns thus the magnetic force applied is constant for a given position of the magnets.

A very interesting feature of the magnetic tweezer is its ability to rotate the magnetic bead, allowing one to easily twist the molecule which tethers the bead to the sample

Magnetic tweezers intrisincally apply a constant force to a molecule whose extension varies in response to this stress. This extension is usually measured by tracking in video the position of the bead using videomicroscopy.