In an autocatalytic chemical reaction a substance $A$ is converted into a substance $B$ in such a manner that
$$
\frac{d x}{d t}=k x(a-x)
$$
where $x$ is the concentration of substance $B$ at time $t, a$ is the initial concentration of substance $A$, and $k$ is a positive constant. Determine the value of $x$ at which the rate $d x / d t$ of the reaction is maximum.