[en] In this paper, the behaviour of the fluctuation of the fluctuation field (H_f) and its relationship with the activation energy for reversal in magnetic systems is examined. At a constant level of magnetisation, the link between thermal energy and field induced changes in the magnetisation of the system using different forms of the equation of state are found to give the same value of H_f. Accordingly, the former S/χ_irr and the ∂H/∂ln(M_irr)|_Mirr techniques for determining H_f are found to be consistent so long as the analysis of H_f is made at the same constant level of irreversible magnetisation (M_irr). Both techniques show that the fluctuation field can be determined using H_f=ΔH/Δln(t)|_Mirr which is more accessible to experiment. Hence, one can define H_f as the change in field required to maintain the magnetisation of the system unchanged during a time interval of Δln(t)=1. The behaviour of the fluctuation field in systems that contain a distribution of activation energies (f(ΔE)) is found to be governed by the nature of f(ΔE). For the case where this distribution arises from distributions of both particle volumes and anisotropy fields, the variation of H_f with M_irr is predicted to be approximately constant when both distributions have comparable standard deviations. Also H_f is predicted to be constant with M_irr is the case where both distributions become extremely narrow (i.e. the case of a single activation energy). Hence H_f remaining constant with M_irr cannot be used to show that the activation energy in the system is constant. In addition if the system contains a single activation energy, a constant value of H_f cannot be used to distinguish between reversal mechanisms such as coherent rotation or weak domain wall pinning