(source: http://www.jove.com/video/51503/magnetic-tweezers-for-the-measurement-of-twist-and-torque)  

Measuring twist and torque
Magnetic tweezers, a powerful single-molecule manipulation technique, can be adapted for the direct measurements of the twist (using a configuration called freely-orbiting magnetic tweezers) and torque (using a configuration termed magnetic torque tweezers) in biological macromolecules. Guidelines for performing such measurements, including applications to the study of DNA and associated nucleo-protein filaments, are given by five researchers of the Nynke Dekker Lab (BN/Kavli Delft) in this publication in the Journal of Visualized Experiments.

Freely-orbiting and magnetic torque tweezers
Single-molecule techniques make it possible to investigate the behavior of individual biological molecules in solution in real time. These techniques include so-called force spectroscopy approaches such as atomic force microscopy, optical tweezers, flow stretching, and magnetic tweezers. Amongst these approaches, magnetic tweezers have distinguished themselves by their ability to apply torque while maintaining a constant stretching force. Here, it is illustrated how such a “conventional” magnetic tweezers experimental configuration can, through a straightforward modification of its field configuration to minimize the magnitude of the transverse field, be adapted to measure the degree of twist in a biological molecule. The resulting configuration is termed the freely-orbiting magnetic tweezers. Additionally, it is shown how further modification of the field configuration can yield a transverse field with a magnitude intermediate between that of the “conventional” magnetic tweezers and the freely-orbiting magnetic tweezers, which makes it possible to directly measure the torque stored in a biological molecule. This configuration is termed the magnetic torque tweezers. The accompanying video explains in detail how the conversion of conventional magnetic tweezers into freely-orbiting magnetic tweezers and magnetic torque tweezers can be accomplished, and demonstrates the use of these techniques. These adaptations maintain all the strengths of conventional magnetic tweezers while greatly expanding the versatility of this powerful instrument.

For more information, see: Lipfert, J., Lee, M., Ordu, O., Kerssemakers, J. W. J., Dekker, N. H. Magnetic Tweezers for the Measurement of Twist and Torque. J. Vis. Exp. (87), e51503, doi:10.3791/51503 (2014).