An $\alpha$-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 $\alpha$-particle and the sulfur ion move in circular orbits of radii $r_\alpha$ and $r_s$, respectively. The ratio $\left(r_s / r \alpha\right)$ is $\_\_\_\_$ .