E-6

Answers to the questions

1. The volmeter draws current from the circuit, although the volmeter has very large resistance. As a result, the voltage across the resistor i.e. V_T measured by the volmeter is less than the one by slid-wire potentiometer.
2. There is no detectable shift in the emf, unless the battery is at the critical point. At the critical point the emf of the battery drops abruptly so that you can observe a detectable shift.
3. Any fluctuation of the power supply may cause the change of V_ab, hence the change of V_ac(see Fig.4-1). It is advisable to recheck the potentiometer setting with the standard cell after each set of measurements.
4. Yes, if the connecting wire is longer than necessary, which means that the wire's resistance is no longer negligible. It is equivalent to add a potential difference to the branch where the cell to be measured is connected. The x in the equation is not the real one that we need. this x is less than we expect.
5. The possible explanation is that the standard cell is used up.

Do not set the power supply voltage too high. V_ab < 2V is appropriate. For old battery, especially for measuring V_T you can set at even lower voltage. You can set V_ab <1V.