The lowest excitation energy of the elements Neon, Mercury, and Argon was determined by analyzing the fundamental properties of the signal structure in the Franck-Hertz experiment. In order to accurately determine the lowest excitation energy a new method proposed in (Rapior 2006) was employed. The main idea is that the spacings between the minima in the Franck Hertz curve increase linearly due to the additional acceleration over the mean free path. Therefore a linear fit was applied to graphs of spacings \(\Delta E\) versus minimum order \(n\). The fit estimated the lowest excitation energies of Neon (\(19.54\pm 1.48eV\)), Mercury (\(4.72\pm.25eV\)), and Argon (\(11.36\pm.38eV\)) accurately within experimental uncertainty.