A circular region in an $x y$ plane is penetrated by a uniform magnetic field in the positive direction of the $z$ axis. The field's magnitude $B$ (in teslas) increases with time $t$ (in seconds) according to $B=a t$, where $a$ is a constant. The magnitude $E$ of the electric field set up by that increase in the magnetic field is given by Fig. $30-55$ versus radial distance $r ;$ the vertical axis scale is set by $E_{s}=300 \mu \mathrm{N} / \mathrm{C}$, and the horizontal axis scale is set by $r_{s}=4.00 \mathrm{~cm}$. Find $a$.