Physics · Current Electricity · NEET
A voltmeter has finite resistance, so a small current I flows through it when connected across a cell. That same current flows through the cell's internal resistance r, causing a voltage drop I*r inside the cell. The voltmeter reads terminal voltage V = EMF - I*r, which is always a little less than the EMF. The bigger the current drawn, the bigger the error.
At the balance (null) point, the potential drop along the potentiometer wire exactly equals the EMF of the test cell. Since the two potentials are equal and oppose each other, there is zero net potential difference in the galvanometer branch, so the galvanometer shows no deflection and no current flows from the test cell. With I = 0, the internal drop I*r = 0, so the wire's PD equals the true EMF.
Yes, at balance it behaves like a voltmeter with infinite resistance. An ideal voltmeter (infinite resistance) draws zero current and reads the exact EMF. A real voltmeter has large but finite resistance, so it draws a tiny current and reads slightly low. The potentiometer achieves the ideal case at the balance point without needing an impossible infinite-resistance meter.
It measures the true EMF because it draws no current from the test cell at balance, so no internal-resistance drop occurs. A voltmeter connected across a working cell measures terminal voltage V = EMF - I*r. This is the key difference NEET tests: potentiometer gives EMF, ordinary voltmeter gives terminal voltage.
A potentiometer is an accurate and versatile device to measure emf because the method involves:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
At the balance point it draws no current from the cell, so there is no I*r drop inside the cell and it reads the true EMF. A voltmeter always draws some current, so it reads terminal voltage, which is a little less than EMF.
Only an ideal voltmeter of infinite resistance would, because it draws zero current. Real voltmeters have finite resistance and draw a small current, so they read slightly less than EMF. A potentiometer reaches this ideal condition at balance.
The null point is where the galvanometer shows zero deflection. Here the potential drop along the wire exactly equals the cell's EMF, so no current flows from the test cell.
Yes. Because no current is drawn at balance, the potentiometer gives the true EMF, and by comparing balance lengths with and without an external load you can find internal resistance accurately without a meter error.
