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\begin{centering}  \textbf{Figure 6}  \end{centering}  \par Spacings $\Delta E$ between the minima and maximum in the Franck-Hertz curve measured with a Mercury tube plotted against minimum order $n$. Although both the minima and maxima are plotted, the minimum values are really the important data points, since they correspond to the excitation energy of the atoms. Experimental results indicate a dependence on the mean free path, since the spacings between the minima appear to increase linearly. Thus the long mean free path limit (Eqn (Equation  5) is likely to be a better approximation of the data.