variable resistance

Jan 2020
What does it mean to put a variable resistance across the battery in a circuit? Please see the figure below:
Apr 2015
Somerset, England
Presumably your gadget comprising F and G in the diagram develop a beam of charged particles which are directed (and focused through the slit) as the incident beam, by the applied voltage.

The applied voltage comes from your battery with the varaible resistors across. Stacking up a number of cells as shown suggests high voltage to control the incident beam.
The variable resistor will have low total resistance, compared to the beam generator focusing plates so the slider, shown as the arrow will not load the baattery supply and a constant voltage will be maintained on the plates. The variable bit allows fine control of the focusing or direction of the incident beam.
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