  We have investigated the temperature dependence of the electrical
conductivity sigma(N,B,T) of nominally uncompensated,
neutron-transmutation-doped ^{70}Ge:Ga samples in magnetic fields up to B=8 T
at low temperatures (T=0.05-0.5 K). In our earlier studies at B=0, the critical
exponent mu=0.5 defined by sigma(N,0,0) \propto (N-N_c)^{mu} has been
determined for the same series of ^{70}Ge:Ga samples with the doping
concentration N ranging from 1.861 \times 10^{17} cm^{-3} to 2.434 \times
10^{17} cm^{-3}. In magnetic fields, the motion of carriers loses time-reversal
symmetry, the universality class may change and with it the value of mu. In
this work, we show that magnetic fields indeed affect the value of mu (mu
changes from 0.5 at B=0 to 1.1 at B \geq 4 T). The same exponent mu'=1.1 is
also found in the magnetic-field-induced MIT for three different ^{70}Ge:Ga
samples, i.e., sigma(N,B,0) \propto [B_c(N)-B]^{mu'} where B_c(N) is the
concentration-dependent critical magnetic induction. We show that sigma(N,B,0)
obeys a simple scaling rule on the (N,B) plane. Based on this finding, we
derive from a simple mathematical argument that mu=mu' as has been observed in
our experiment.
