We report resistivity and thermopower measurements on a series of Pb-substituted ceramic samples of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$. The samples show an increase in their conductivity and a systematic fall to more negative values of thermopower as holes are added with the introduction of progressively more Pb or oxygen, trends held in common with other high-${\mathit{T}}_{\mathit{c}}$ superconductors. Furthermore, the thermopower falls linearly with increasing temperature in all of our samples, with the unusual property that it extrapolates to a large (typically >5 μV/K) nonzero value at zero temperature. This offset value decreases dramatically as holes are added, whereas the linear slope shows a much weaker decrease. This temperature dependence can be fitted to a model in which the thermopower is strongly enhanced at low temperatures by the electron-phonon interaction.

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