Hydrogen atom is bombarded with electrons accelerated through a potential difference of $\mathrm{V}$, which causes excitation of hydrogen atoms. If the experiment is being performed 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 $\alpha=$ __________.