The \appendix command changes the way sectional units are numbered. The \appendix command generates no text and does not affect the numbering of parts.
Thus, if you want your appendix to be set off with a section-like label you need something like
\begin{center} {\bf APPENDIX} \end{center}
One can create a section labeled "APPENDIX" with distinctive numbering by using the \section command with suitably redefined commands for the Counters. The following example generates a single "Appendix" with equations numbered A-1, A-2, ...
\renewcommand{\theequation}{A-\arabic{equation}} % redefine the command that creates the equation no. \setcounter{equation}{0} % reset counter \section*{APPENDIX} % use *-form to suppress numberingIf one had several appendices, appropriate numbering could be obtained by redefining the \thesection command and resetting the section counter in an analogous manner.