Electric Field linearly accelerates $e^{-}$ from voltage in Electron Gun: Va
Magnetic Field circularly accelerates $e^{-}$ from current in Helmholtz coils through coils: Iα
Picture of the real experiment for deflection of electrons in a magnetic field
Grid for data logging

Use the experiment to measure the ratio of the electrons charge to it's mass.

Fill the table with different acceleration voltages Va respectively coil currents I and the radius you will see in the experiment. So you can determine the charge to mass ratio.
$V_{\text a}~ \text {in V} $ $\Rightarrow v_0~ \text {in}\frac{\text m}{\text s} $ coil current
$I~\text {in A}$
$\Rightarrow B \text{ in T}$ Radius r
in cm
$\frac{e}{m}=\frac{2\cdot V_{\rm{a}}}{\left(B\cdot r\right)^2}~\text{in}\frac{\text C}{\text {kg}}$
$0$ $0$ $-$
$0$ $0$ $-$
$0$ $0$ $-$
$0$ $0$ $-$
$0$ $0$ $-$