Electric Field linearly accelerates $e^{-}$ from voltage in Electron Gun: Va
Magnetic Field circularly accelerates $e^{-}$ from current in Helmholtz coils through coils: Iα

Schematic Diagram

Schematic layout of the experiment about electron deflection in a magnetic field

Check the validity of the formula ${r=\frac{m_e\cdot v_0}{e\cdot B}}$ for the radius of the circular path using the experiment.

The theoretical circular path is shown in blue and can be switched on and off.
Picture of the real experiment for deflection of electrons in a magnetic field
Grid for data logging
$${r = \frac {m_e\cdot v_0}{e\cdot B}}=4.25\,\rm{cm}$$