Figure 1 depicts a circuit consisting of two power supplies, a current meter, and a resistor. The output of the first supply varies, producing different values of $V1$. The output of the second supply, $V2$, which is inverted with respect to the first, is constant. The current meter, which has zero resistance, measures the current $I$.
Figure 2 shows the potential $V1$ as a function of the current $I$ through the circuit.
What is the resistance $R$ of the resistor?