A change in magnetic field H quenches the barriers in a spin glass for heights less than an associated change in Zeeman energy, E_z. The shift of the peak of S(t)=-dM_{TRM}(t,t_w)/H/{d lnt} with H generates E_z={N_s}{X_fc}{H^2}, with X_fc the field cooled magnetic susceptibility per spin, and N_s the number of spins participating in barrier quenching (and barrier hopping). Experiments on Cu:Mn 6at.% and CdCr_{1.7}In_{0.3}S_4 for ranges of H, T, and waiting times t_w generate the correlation length, Ksi(t_w,T) ~ {N_s}^{1/3}, fitted by both the hierarchical model, Ksi(t_w,T)=0.635(t_w}/tau_0)^{0.169T/T_g}, in numerical accord with simulations, or the droplet model, Ksi(t_w,T)={10^{-5}}{{[(T/T_g)Ln(t_w/tau_0)]}^{1/0.21}}, with too small a prefactor for simulation time scales, and exponent 1/psi at the lower limit for psi.