A) Calibrating a voltmeter using a potentiometer is as follows:

1. Connect the potentiometer to a power source and adjust its resistance to produce a known voltage.
2. Connect the voltmeter in parallel with the potentiometer, so that it measures the voltage across the potentiometer.
3. Compare the reading of the voltmeter with the known voltage, and if necessary, adjust the voltmeter's sensitivity until it gives a correct reading. This can be done by adjusting the internal resistance of the voltmeter or by using a multiplier resistor.
4. Repeat the procedure for different values of voltage to ensure the voltmeter is calibrated over its entire range.
   

Note: The above procedure is a general outline and may vary depending on the specific voltmeter being used. It is important to consult the manufacturer's instructions before attempting to calibrate the voltmeter.

B) Calibrating an ammeter using a potentiometer is as follows:

1. Connect the potentiometer to a power source and adjust its resistance to produce a known voltage.
2. Connect the ammeter in series with the potentiometer, so that it measures the current flowing through the potentiometer.
3. Adjust the ammeter's sensitivity until it gives a correct reading of the current. This can be done by adjusting the ammeter's internal resistance or by using a shunt resistor.
4. Repeat the procedure for different values of current to ensure the ammeter is calibrated over its entire range.
   

Note: The above procedure is a general outline and may vary depending on the specific ammeter being used. It is important to consult the manufacturer's instructions before attempting to calibrate the ammeter.