Fig. 3. Measurement of the lateral force in square glass capillaries. (A) A cartoon of the geometric representation of extension measurements in lateral pulling when using a square glass capillary tube. When the capillary is tilted ∼5°, the extension is simply l = x – R. (B) Position coordinates (x, y, z) and extension (l) of a DNA molecule measured in a lateral pulling cycle in a square capillary where the force is increased by approaching the magnet to the closest position (t = 20 s) and then decreased stepwise. Note that in glass capillaries, there is no peak in x as the bead is permanently in contact with the surface, and z measurement is close to zero throughout the entire measurement.