The critical conductivity exponent of Si:P changes from near 1/2 in ze
ro field to 0.86 +/- 0.15 in a magnetic field of 8 T, consistent with
the theoretical expectation of 1. According to recent theory, similar
behavior found earlier in Si:B, where spin-orbit scattering is strong,
corresponds to the universality class for magnetic impurities. These
measurements in Si:P thus constitute a clear determination of the crit
ical conductivity exponent near the metal-insulator transition in the
universality class for high magnetic field.