If a null-point potential measurement device has a current meter capable of detecting a microamp of current,

Question:

If a null-point potential measurement device has a current meter capable of detecting a microamp of current, what is the maximum resistance that it can have in order to keep the error in the voltage (between point A and point B in Figure 2.1) $\leq 0.05\mathrm{mV}$ ?

Figure 2.1

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