The correct answer is: A. necessary to standardise the potentiometer.
A potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If one terminal is connected to a fixed voltage, the voltage across the resistor can be changed by sliding the contact. This makes a potentiometer useful for measuring voltage, current, and resistance.
To measure resistance with a potentiometer, the potentiometer is first calibrated by connecting it to a known voltage source. The sliding contact is then adjusted until the voltage across the resistor is equal to the known voltage. The resistance of the resistor can then be calculated from the known voltage and the voltage across the resistor.
It is necessary to standardize the potentiometer before using it to measure resistance. This is because the resistance of the potentiometer can change over time due to factors such as temperature and humidity. Standardizing the potentiometer ensures that it is always accurate.
Option B is incorrect because it is necessary to standardize the potentiometer before using it to measure resistance. Option C is incorrect because a volt ratio box is not necessary to measure resistance with a potentiometer. Option D is incorrect because option A is the correct answer.