Fig. 3. A schematic of the device used for measuring frictional forces between the file (dark red) and plectrum (pink). The plectrum was mounted on a linear stage, which moved the plectrum over the file at velocity (V) (directed into/out of the page). A force beam was mounted on the vertical beam to measure the frictional force, F_f, which is directed opposite to the movement of the plectrum. A second force beam was mounted on a manual translation stage with which the normal force (F_n), applied orthogonally between the two surfaces, could be varied. The inset shows the directions of the force vectors; note that this drawing is rotated 90° relative to the apparatus diagram.