A long excitation coil piercing a superconducting (SC) ring is used to
generate ever increasing persistent current in the ring, until the current
destroys the order parameter. Given that the penetration depth λ is
known, this experiment measures, hypothetically, the coherence length ξ,
hence the name "Xiometer". We examine various aspects of this theoretically
driven hypothesis by testing niobium rings with different dimensions, and by
comparing the results to the known values of ξ. We then apply the method to
two La1.875​Sr0.125​CuO4​ rings at T→0. In one, the
current flows in the CuO2​ planes hence it is set by ξab​. In the
other, the current must cross planes and is determined by ξc​. We find
that ξc​=1.3±0.1~nm, and ξab​<2.3 nm indicating that at low
temperatures the Cooper pairs are three dimensional.Comment: 6 pages with 7 figure