  We present the first measurement of a non-zero magnetic field in the
eclipsing material of a black widow pulsar. Black widows are millisecond
pulsars which are ablating their companions; therefore they are often proposed
as one potential source of isolated millisecond pulsars. PSR J2256-1024 is an
eclipsing black widow discovered at radio wavelengths and later also observed
in the X-ray and gamma parts of the spectrum. Here we present the radio timing
solution for PSR J2256-1024, polarization profiles at 350, 820, and 1500~MHz
and an investigation of changes in the polarization profile due to eclipsing
material in the system. In the latter we find evidence of Faraday rotation in
the linear polarization shortly after eclipse, measuring a rotation measure of
0.44(6) rad per meter squared and a corresponding line-of-sight magnetic field
of 3.5(17) mG.
