How it works
A voltmeter is a measurement instrument used to measure the electrical potential difference (voltage) between two points in a circuit. To measure the voltage across a component, you must connect the voltmeter in parallel with that component.
An ideal voltmeter has infinite internal resistance, meaning it draws absolutely zero current from the circuit and therefore does not alter the circuit's behavior while measuring. In practice, real voltmeters have a very high (but finite) resistance, typically 10 megaohms.