A universal amplitude ratio for the q<4 Potts model from a solvable lattice model

The universal amplitude ratio $R_{\xi}$ for the ($q\leqslant 4$)-state Potts model in two dimensions is determined by using results for the dilute A model in regime 1. The nature of the relationship between the Potts model and the dilute A model, both related to $\phi_{2,1}$ perturbed conformal field theory, is discussed.