clarify three stage pot div

pt100/pt100.tex

@@ -52,7 +52,9 @@ look-up tables or a suitable polynomial expression.
 \end{figure}
 
 
-The voltage ranges we expect from this three stage potential divider
+The voltage ranges we expect from this three stage potential divider\footnote{
+two stages are required for validation, a third stage is used to measure the current flowing
+through the circuit to obtain accurate temperature readings}
 are shown in figure \ref{fig:pt100vrange}. Note that there is
 an expected range for each reading, for a given temperature span.
 Note that the low reading goes down as temperature increases, and the higher reading goes up.