  Experimental results of studying the effect of a weak magnetic field ($\sim
$300 Gs) on the intensity of the terahertz emission ($\lambda \approx $100 $\mu
$m) of hot electrons in $n$-Ge (crystallographic orientation $< 1,0,0 >)$ at
helium temperatures ($T\sim $5 K) are presented and discussed. It is shown that
the strong influence of this field (decrease of the emission intensity by
500$\div $1000{%}) is related to a decrease of the carrier concentration at
weak electric fields and the appearance of the magnetoresistance at stronger
fields. The longitudinal magnetoresistance becomes significant due to the
anisotropy of the energy dispersion law of electrons and a strong deformation
of the electron velocity distribution function by the electric field (which is
beyond the framework of the diffusion approximation).
