The following table shows the screen current reading from the JEOL JEM 3200FS's large phosphor screen over time (starting in the fall of 2014). We had been experiencing frequent and significant changes in the strength of the electron beam and decided to track this more closely (rather than simply noticing one day that the "beam seems to be weaker than it used to be"). We (both the JEOL service engineers and the staff) adjusted the A1 and A2 extraction voltages for the FEG in order to adjust for the fluctuation in beam current.
David Morgan's routine alignment procedure involves recording a series of standard images, and he began to record the beam current reported for the first image. The image of a standard replica diffraction grating (waffle grid) is acquired at a magnification of 200,000x with the electron beam completely filling the large (8 cm radius) phosphor screen.