Fig. 2. Measurement of the force in lateral magnetic tweezers in regular flow cells. (A) A cartoon of the geometric representation of extension (l) measurements in vertical pulling, considering the DNA molecule is attached to the lowest part of the bead. (B) A cartoon of the geometric representation of extension measurements in lateral pulling. The extension is computed as . This assumes that the DNA is attached analogously to vertical pulling. (C) Position coordinates (x, y, z) and extension (l) of a DNA molecule measured in a lateral pulling cycle, where the force is first suddenly increased (by moving the magnet to the closest position) (t = 20 s) and then decreased, in a stepwise manner. This produced a fast increase of the x signal and a small peak (arrow) that occurs just before the lift-off of the bead from the surface (left dashed line). The bead rests again at the surface beyond t = 95 s where the z measurement is close to zero (black arrow, and right dashed line), and the x coordinate recovers its maximum value.