Calculating the Conformal Charge from the Second Moment of the Spin-Spin Correlation Function

We develop high field expansions for the second moment of the spin-spin correlation function of the two-dimensional Ising model at arbitrary temperature T. Setting T = T_c and approaching the critical point along the field direction, this quantity has a 1/h² divergence as the external field h goes to zero. The amplitude for this divergence is estimated by series extrapolation methods, and is shown to lead to an estimate for the central charge of 0.50 by using Cardy's formula for the central charge in terms of hyperuniversal amplitude ratios.