Figure 13.2.

Force calibration. Force as a function of the magnets position (Z_mag) measured for several micron-sized beads (Myone Invitrogen) attached by a single λ-DNA molecule (each color corresponds to a separate bead). Note that the position Z_mag is measured with respect to the sample chamber (e.g., Z_mag = 0 when the magnets touch the upper surface of the chamber and when the magnets are 1 mm above the chamber Z_mag = −1). The blue line corresponds to a fit of the data to the function $F=F_{\text{max}}\text{exp}({\mathit{\text{AZ}}}_{\text{mag}}+{\mathit{\text{BZ}}}_{\text{mag}}^2)$, which yields F_max = 21 pN, A = 3.53 mm⁻¹ and B = 0.66 mm⁻². This curve can be used to estimate the force on the bead at a known position of the magnets.