Biot-Sawart law indicates that the moving electrons (velocity $\overrightarrow{v}$) produce a magnetic field $\overrightarrow{B}$ such that 

According to Biot-Savart law, $d\overrightarrow{B}=\frac{{\mu }_0}{4\pi }\frac{{\displaystyle i}\left(\overrightarrow{dl}\times \overrightarrow{r}\right)}{r^3}$

$\Rightarrow d\overrightarrow{B}=\frac{{\mu }_0}{4\pi }\frac{{\displaystyle \left(\frac{dq}{dt}\right)\left(\overrightarrow{dl}\times \overrightarrow{r}\right)}}{r^3}$

$\Rightarrow d\overrightarrow{B}=\frac{{\mu }_0}{4\pi }\frac{{\displaystyle e\left(\frac{\overrightarrow{dl}}{dt}\times \overrightarrow{r}\right)}}{r^3}$

$\Rightarrow d\overrightarrow{B}=\frac{{\mu }_0}{4\pi }\frac{{\displaystyle \left(dq\right)\left(\overrightarrow{v}\times \overrightarrow{r}\right)}}{r^3}$

It indicates that the moving electrons (velocity $\overrightarrow{v}$) produce a magnetic field $\overrightarrow{B}$ such that $\overrightarrow{B }\perp \overrightarrow{v}$.