Some metal alloy is known to have electrical conductivity and electron mobility values of $1.2 \times 10^{7}(\Omega-\mathrm{m})^{-1}$ and $0.0050 \mathrm{m}^{2} / \mathrm{N}-\mathrm{s}$, respec-
tively. Through a specimen of this alloy that is $35 \mathrm{mm}$ thick is passed a current of $40 \mathrm{A}$. What magnetic field would need to be imposed to yield a Hall voltage of $-3.5 \times 10^{-7} \mathrm{V} ?$