Universality of Second-Order Phase Transitions: The Scale Factor for the Correlation Length

A universality hypothesis relates the amplitude of the singular parts of the coherence length and the specific heat (or surface tension). For the spin-\textonehalf{} Ising model it is exact in two dimensions and numerically accurate to within 1% in three. It is consistent with measurements on the "Ising-like" systems C${\mathrm{O}}_{2}$, Xe, and $\ensuremath{\beta}$-brass and the "Heisenberg-like" systems RbMn${\mathrm{F}}_{3}$ and EuS to within experimental uncertainties (\ensuremath{\sim} 20%). It provides a sensitive and experimentally convenient indicator of symmetry ("universality") class.