Final answer:
A multimeter set as a voltmeter should be connected in parallel with a resistor to accurately measure the potential difference across it, ensuring the multimeter's resistance is high enough not to affect the circuit substantially.
Step-by-step explanation:
To measure the potential difference across a resistor, a multimeter, specifically set to measure voltage, must be connected in parallel with the resistor. This is because you want to measure the voltage present across the resistor without significantly altering the circuit's current flow. A voltmeter has a high resistance, which ensures that only a very small current flows through it when connected in parallel, so the circuit's behavior remains largely unaffected.
Furthermore, if the voltmeter had a resistance that was comparable to that of the component it's measuring, it could introduce significant errors to the measurement by affecting the circuit's overall resistance. This underlines the importance of using a voltmeter with a resistance that is significantly higher than that of the component being tested to ensure accurate readings.