Magnetic Tweezers Here, a single molecule is attached on one end to a glass surface and at the other end to a magnetic bead. A magnetic field (produced by a pair of permanent magnets) attracts the bead (the force acting on the bead depends on the distance between the bead and the magnets). Using a slightly coherent source of light (that generates diffraction rings around the beads), the x, y and z positions of the beads can be determined with sub-nanometer accuracy. Subsequently, the force acting on the bead is measured from the variance of the x and y positions. The magnetic field is homogeneous over large distances (mm, parallel to the glass slide) and varies slowly with the distance (perpendicular to the glass). Therefore, multiple beads (up to hundreds of beads) can be studied in parallel at a constant force. While force versus extension curves can be measured, MT are best suited to study molecular motors (at a constant force) or enzymes that have a topological effect on DNA. MORE.