Hydrogen atom is bombarded with electrons accelerated through a potential different of $V$, which causes excitation of hydrogen atoms. If the experiment is being formed at $\mathrm{T}=0 \mathrm{~K}$. The minimum potential difference needed to observe any Balmer series lines in the emission spectra will be $\frac{\alpha}{10} \mathrm{~V}$, where $\alpha=\quad$