The mass and charge of an electron are m and e respectively. The momentum acquired by the electrons in l cm of wire when a current i starts to flow is 

If $\mathrm{\alpha }$ be the area of cross-section of wire, then its volume is a l. Let n be the number of electrons per unit volume. Now, the number of electrons in the wire is n a L The mass of the electron is m and if its drift velocity be v_d, then momentum acquired by the electrons is the momentum acquired by the wire. So, momentum acquired by the wire
$\mathrm{p}={\mathrm{nalmv}}_{\mathrm{d}}$
We know that ${\mathrm{v}}_{\mathrm{d}}=\mathrm{i}/\mathrm{nae}$
$\therefore \mathrm{p}=\mathrm{nalm}(\mathrm{i}/\mathrm{nae})$
or $\mathrm{p}=\frac{\mathrm{mil}}{\mathrm{e}}$