I think you'd be correct on the use/or addition of a potentiometer to current control in Colloidal Silver circuit. Though keep in mind that a resistor will not only limit current but voltage as well, and that this could add a challenge to the startup phase, as the voltage is one of the main properties used to overcome the limited conductivity of the cell - if this makes any sense
In addition to this, and I don't mean to pick on this particular idea, though I can think of another challenge that could come from the use of a potentiometer in that it would not allow for repeatability within a process. And while I wouldn't go as far as saying that this be necessary, I do think however, that there are benefits to having consistency in terms of time and rates(volts/current) during processing. As this allows for control and monitoring.
One example of this would be with a setup which would provide volt and current measurements over time. This could in-turn, provide the means to adjust certain parameters during processing. ie, you might start a batch and let it run for 30mins, after which, a timer could tell you to either; change-out or clean your cathode(s) for example.
Whatever the case, and I'm certainly not saying the use of a potentiometer is somehow a bad thing, though I do however think it it may be worth considering some of the possibilities that come with the use of the different types of current control measures used in the production of Colloidal Silver .