A particle of charge $-16\times {10}^{-18} \mathrm{C}$ moving with velocity $10 \mathrm{m} {\mathrm{s}}^{-1}$ along the $x$-axis enters a region where a magnetic field of induction $B$ is along the $y$-axis and an electric field of magnitude ${10}^4 \mathrm{V} {\mathrm{m}}^{-1}$ is along the negative $z$-axis. If the charged particle continues moving along the $x$-axis, the magnitude of $B$ is