I often see circuits like the one below, in questions which ask where the ammeter and voltmeter should be placed to correctly calculate the resistance. But every time, I see a variable resister placed at the end of the circuit. What is the purpose of the variable resistor in the circuit I understand that it allows someone to manually change the resistance in the circuit, but why is it necessary in the circuit?
The variable resistor is used to vary the current P in the circuit. This enables you to obtain several pairs of values of P & Q, so that you can plot a straight-line graph of Q vs P to get R = slope. This technique takes an average of the values of resistance for each pair of P & Q, and allows you to check that Ohm's Law (V=IR or Q=IP) is valid.