Figures 51 and 52 in the book show how to use an oscilloscope 2AP1 CRT to measure e/m using Hoag’s method. The pictures in this figure supplement the book’s, showing you how to construct the d.i.y. setup, as well as the way in which the electron beam fan is reduced to a point as the magnetic field fulfills the focusing equation. In this setup, an AC signal is placed across one set of plates of the CRT to produce a line on the screen. The solenoid is then energized until the line makes one complete helical turn.
e/m can then be calculated from the magnetic field B needed to cause the line to rotate one full turn until it disappears into a point: 