As a variable resistor's resistance can be changed, we can therefore change the current in the circuit. This will be according to Ohms Law, as a simple resistor is a Ohmic Conductor.

So if we set up a circuit with a simple cell and a variable resistor with a Ammeter in series with resistor and a Voltmeter in parallel with the resistor, if we change the resistance, the current will change causing a change in potential difference across the resistor. So if we can collect data of both voltage and current for different resistance value, a graph can be plotted. This will give us the Ohmic relationship between I and V.