  Using X-ray photoelectron spectroscopy, a systematic study is performed of
the doping dependence of the chemical potential $\mu$ in two systems of
(Bi,Pb)$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$. The doping is varied by changing
the O content of the sample. The measured shifts of the chemical potential are
compared with present models for the doping behaviour of $\mu$ in high-$T_{\rm
c}$ materials. VSGD.94.6.3
