 Transverse-fluctuation calibration of a tensioned tether

For a tether with extension $\ell$ and tensile load $F$, a small transverse coordinate costs $F{x}^2/(2\ell )$. Equipartition gives $F=k_{B}T\ell /⟨x^2⟩$ for one Cartesian transverse coordinate. Using the sum of two transverse variances changes the factor of two. Geometry and measurement filtering must match the calibration model.