An $\mathrm{\alpha }$ -particle (mass 4 amu) and a singly charged sulfur ion (mass 32 amu) are initially at rest. They are accelerated through a potential V and then allowed to pass into a region of uniform magnetic field which is normal to the velocities of the particles. Within this region, the $\mathrm{\alpha }$ -particle and the sulphur ion move in circular orbits of radii ${\mathrm{r}}_{\mathrm{\alpha }}$ and ${\mathrm{r}}_{\mathrm{S}}$ , respectively. The ratio $\left({\mathrm{r}}_{\mathrm{S}}/{\mathrm{r}}_{\mathrm{\alpha }}\right)$ is _______