Figure 54 in the book shows our setup based on a 6AF6-G “magic eye” tuning tube to measure e/m. The pictures in this figure supplement the book’s Figure 54 to help you build your own system. In the 6AF6, electrons produced by a thermionic cathode cause fluorescence on the tube’s anode. Applying an external magnetic field curves the path of the electrons reaching the anode’s fluorescent coating. Knowing R and the voltage applied to the tube allows one to calculate e/m.