With the accelerating voltage at an intermediate value, the current in the Helmholtz coils may be adjusted so that the outside edge of the beam strikes the outside edge of each bar in turn. The outer edge of the beam is used because it is determined by the electrons which have the highest velocity. The electrons which leave the negative end of the filament fall through the greatest potential difference between filament and cylinder and thus have the greatest velocity. The maximum potential is the one measured by the Plate Voltage Meter.
In addition, electrons colliding with mercury atoms lose energy and travel in new orbits with smaller radii. Thus, the outer edge of the beam is actually the locus of the orbits of electrons traveling at maximum speed.
Repeat the measurement of field current I against orbit radius r for several values of the plate voltage V. For one measurement, test the reproducibility of the current setting as an aid to error analysis.
The tube manufacturer supplies the following values for the distances from the filament to the outside of each bar in succession:
6.48 cm, 7.75 cm, 9.02 cm, 10.30 cm, 11.54 cm