A potentiometer uses the Null Point method.

• At the balancing point, the potential from the secondary circuit exactly matches that from the potentiometer wire segment.
• Consequently, no current flows from the test cell ($I=0$).
• Since $V=E-Ir$ and $I=0$, the potentiometer measures the actual EMF ($E$) rather than the terminal voltage ($V$). This makes it a primary measurement tool unlike a voltmeter which always draws some current.