A stream of a positively charged particles having ${q \over m} = 2 \times {10^{11}}{C \over {kg}}$ and velocity ${\overrightarrow v _0} = 3 \times {10^7}\widehat i\,m/s$ is deflected by an electric field $1.8\widehat j$ kV/m. The electric field exists in a region of 10 cm along $x$ direction. Due to the electric field, the deflection of the charge particles in the $y$ direction is _________ mm.