  We present a new method for direct measurement of magnetic fields on
ultracool stars and brown dwarfs. It takes advantage of the Wing-Ford band of
FeH, which are seen throughout the M and L spectral types. These molecular
features are not as blended as other optical molecular bands, are reasonably
strong through most of the spectral range, and exhibit a response to magnetic
fields which is easier to detect than other magnetic diagnostics, including the
usual optical and near-infrared atomic spectral lines that have heretofore been
employed. The FeH bands show a systematic growth as the star gets cooler. We do
not find any contamination by CrH in the relevant spectral region. We are able
to model cool and rapidly-rotating spectra from warmer, slowly-rotating spectra
utilizing an interpolation scheme based on optical depth scaling. We show that
the FeH features can distinguish between negligible, moderate, and high
magnetic fluxes on low-mass dwarfs, with a current accuracy of about one
kilogauss. Two different approaches to extracting the information from the
spectra are developed and compared. Which one is superior depends on a number
of factors. We demostrate the validity of our new procedures by comparing the
spectra of three M stars whose magnetic fluxes are already known from atomic
line analysis. The low and high field stars are used to produce interpolated
moderate-strength spectra which closely resemble the moderate-field star. The
assumption of linear behavior for the magnetic effects appears to be
reasonable, but until the molecular constants are better understood the method
is subject to that assumption, and rather approximate. Nonetheless, it opens a
new regime of very low-mass objects to direct confirmation and testing of their
magnetic dynamos.
