A generic instrument for force and torque uses a magnetically responsive bead in a controlled magnetic field, with optical tracking. Translational force requires a field gradient. Permanent or anisotropic moments can feel orienting torque; an isotropic induced moment parallel to the field has no such torque. This definition concerns the physical instrument, without biological applications.
For a tether with extension and tensile load , a small transverse coordinate costs . Equipartition gives for one Cartesian transverse coordinate. Using the sum of two transverse variances changes the factor of two. Geometry and measurement filtering must match the calibration model.
For an equilibrium overdamped harmonic coordinate with covariance , a camera exposure records the average over that interval. Integrating the covariance twice gives . The displayed reduction follows with . In the tether calibration , ignoring exposure averaging overestimates the inferred force. Independent localization-noise variance instead adds to the measured variance and biases it in the opposite direction. Other filters require their actual response functions.
For a linear isotropic unsaturated effective susceptibility contrast , the induced-moment energy is , giving . The one-half differs from the fixed permanent-dipole energy because the applied field creates the moment. Demagnetization and saturation are absorbed into or replace the assumed response model.
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Magnetic tweezers are a powerful experimental technique used primarily in biophysics and molecular biology to manipulate and study the properties of individual biomolecules, such as DNA, RNA, and proteins. This technique utilizes magnetic fields to exert forces on magnetic beads that are attached to or associated with the molecules of interest. ### Key Components of Magnetic Tweezers: 1. **Magnetic Beads**: These beads, often coated with a biomolecule, serve as handles for manipulating the molecule of interest.