= Velocity-selected magnetic mass analyzer
Reuse the <velocity selector> analysis in the <Thomson charge-to-mass experiment>, but now send positive <ions> with $q=ze$ into a separate magnetic analyzer. The selector fields $E_s$ and $B_s$ give $v_0=E_s/B_s$. If the analyzer field $B_a$ is perpendicular to the selected <velocity>, its <gyroradius> gives
$$
\frac{m}{q}=\frac{B_a r}{v_0}=\frac{B_aB_s r}{E_s},\qquad D=2r
$$
for a semicircular return to the entrance plane. The displayed impact separation $D$ is a diameter. This formula is linear in $r$ at fixed selected <speed>; the fixed-<electric potential difference> model above instead gives $m/q\propto r^2$. They impose different experimental conditions.
For a chosen example, take $E_s=2.00\times10^4\,\mathrm{V/m}$, $B_s=0.200\,\mathrm T$, and $B_a=0.500\,\mathrm T$. Thus $v_0=1.00\times10^5\,\mathrm{m/s}$. Approximating an <ion> <mass> by $24\,\mathrm u$ and setting $q=e$ gives $r\approx49.75\,\mathrm{mm}$ and $D\approx99.50\,\mathrm{mm}$. Here $\mathrm u$ denotes the <unified atomic mass unit>. These are stipulated teaching inputs, not a reported Aston measurement or an exact <magnesium> <atomic mass>.
A finite accepted <speed> range broadens the radius distribution. At fixed $m/q$ and $B_a$, $\delta r/r=\delta v/v$. At fixed <electric charge> and <mass>, $K=mv^2/2$ gives $\delta v/v\simeq\delta K/(2K)$ for a small <kinetic energy> spread. A one-percent <kinetic energy> spread therefore produces roughly a half-percent radius spread before other broadening is included. Its width convention must be specified before converting that statement into a <mass resolving power>.
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