An ion source in a mass spectrometer produces a beam of ions of mass $M$ and charge $Q$. These ions are accelerated from rest through an electrostatic drift chamber across a potential difference $U$, emerging with a terminal speed $u$.

A second species of ion, having a mass of $\frac{M}{2}$ and a charge of $5Q$, is accelerated from rest through a potential difference of $3U$ in the same apparatus.