The coefficient of resistivity due to electron-phonon interactions in the normal state is also given in terms of λ . For example for temperatures comparable to an larger than ω , the resistivity due to e-ph interactions in a metal may be written in terms of the scattering rate Study of empirical relations among the parameters determining Transitions temperatures in sup erconductors gives useful insights to the physics of metals. In his analysis of the B. Empirical Relations in transition metal superconductivity τ − $^{1}$( T ) = 2 πλ e − $^{ph}$T. (26) FIG. 5: Empirically determined ”constancy” of N (0) < I $^{2}$> in classes of metals an d compounds. properties of superconducting metals and compounds, McMillan [25] noted that N (0) < I 2 > varies only by about factor of 2 while N (0) and < I 2 > vary by a factor of about 10. However the empirically ”constant” N (0) < I 2 > has different values for different classes of metals and compounds. For example, see Fig. (5), it is is close to one value for the pseudo- potential metals like Sn, Pb, Bi and their alloys and another for the transition metals and their alloys and yet another in the A-15 compounds. McMillan proffered no explanation for the transition metals but showed using the fact that the ion-plasma frequency Ω$_{p}$ are always much larger than the phonon frequencies, that for the pseudopotential metals that N (0) < I 2 > / Ω 2 p is approximately constant. 28 GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> GLYPH<SolidCircle> Nb$_{3}$ Sn W Mo Nb Ta V Sn Α GLYPH<Minus>$_{Ga}$ Tl In$_{0.67}$ Tl$_{0.33}$ In Pb$_{0.7}$ Bi$_{0.3}$ Hg In$_{2}$ Bi Bi$_{2}$ Tl Β GLYPH<Minus>$_{Ga}$ Pb$_{0.45}$ Bo$_{0.55}$ Pb$_{0.8}$ Bi$_{0.2}$ Pb GLYPH<Multiply> GLYPH<Multiply> 0.5 1 1.5 2. 2.5 1 2 3 4 5 6 7 8 Λ N 0 GLYPH<Less> I 2 GLYPH<Greater> eV GLYPH<Angstrom> 2