[texhax] capital cursive A?

[John Wheeler]

I apologize in advance if this is available in the faq-s or documentation,
but I've searched in vain, including asking others privately and searching
the amsmath documentation.
I've long wanted a capital cursive A --- not the curly script-like capital A
but the one we were taught to make in grade school; like the lower case cursive
a but the size of a capital letter: a distorted oval with a tail at the lower
right --- for use as a symbol in physical chemistry for the affinity that
wouldn't be so easily confused with other capital A-s that float about.
In desperation, I came up with the following kluge that causes even me to 
shudder, and will clearly get me into trouble with the automatic sizing 
mechanisms of LaTeX, as witnessed by the final equation in the example below.
Can someone help me find the right way to generate this character?  At least
on my installation, the first line and first two displayed equations look fine
(but I'd much rather accomplish this in an approved manner, consistent with
the expectations of LaTeX) and the last displayed equation makes the
A in the derivative much too big, although the subscripted A is about right.
Thanks, in advance, for any help,
John Wheeler
jcw at chemj2.ucsd.edu
EXAMPLE:
\documentclass[12pt]{article}
\usepackage{amsthm,amsbsy,amssymb,amsfonts,amsmath,amscd}
\usepackage{mathrsfs}
\usepackage{latexsym}
\usepackage{fontenc}
\usepackage[mathscr]{euscript}
\newcommand{\Aff}{\text{\Large${a}$}}
\newcommand{\sAff}{\text{\normalsize ${a}$}}
\begin{document}
{$\Aff$}, $B_{\sAff}$
$$
\left(\frac{\partial \Aff}{\partial T}\right)_{P,\xi} = 
\left(\frac{\partial S}{\partial \xi}\right)_{P,T}
$$
$$
\left(\frac{\partial \xi}{\partial T}\right)_{P,\sAff} =
-\left(\frac{\partial S}{\partial \Aff}\right)_{P,T}
$$
$$
\frac{\left(\frac{\partial \xi}{\partial T}\right)_{P,\sAff}}
{\left(\frac{\partial S}{\partial \Aff}\right)_{P,T}} = -1
$$
\end{document}