  Measurements of the Hall conductivity in MnSi can provide incisive tests of
theories of the anomalous Hall (AH) effect, because both the mean-free-path and
magnetoresistance (MR) are unusually large for a ferromagnet. The large MR
provides an accurate way to separate the AH conductivity $\sigma_{xy}^A$ from
the ordinary Hall conductivity $\sigma_{xy}^N$. Below the Curie temperature
$T_C$, $\sigma_{xy}^A$ is linearly proportional to $ M$ (magnetization) with a
proportionality constant $S_H$ that is independent of both $T$ and $H$. In
particular, $S_H$ remains a constant while $\sigma_{xy}^N$ changes by a factor
of 100 between 5 K and $T_C$. We discuss implications of the hidden constancy
in $S_H$.
