  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 $\lambda$ is
known, this experiment measures, hypothetically, the coherence length $\xi$,
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 $\xi$. We then apply the method to
two La$_{1.875}$Sr$_{0.125}$CuO$_4$ rings at $T \rightarrow 0$. In one, the
current flows in the CuO$_2$ planes hence it is set by $\xi_{ab}$. In the
other, the current must cross planes and is determined by $\xi_{c}$. We find
that $\xi_{c}=1.3 \pm 0.1$~nm, and $\xi_{ab}<2.3~$nm indicating that at low
temperatures the Cooper pairs are three dimensional.
