We report on detailed measurements of the electrical conductivity sigm
a(T) in uncompensated Si:P in a magnetic field B of 6T between 30 mK a
nd 1K. The approximate relation sigma(T) = sigma(0) + m(B) square-root
T allows a reliable extrapolation of sigma(0). The critical behaviour
sigma(0) approximately (N - N(c))mu' close to the critical P concentr
ation N(c) yields a critical exponent mu' almost-equal-to 1.3, with a
tendency to smaller values at a crossover concentration which is close
to the concentration where m(B) changes strongly. The implications fo
r the previously proposed solution to the exponent puzzle for B = 0 ar
e discussed.