Experiment 4 - The Voltage Measurement

How do you measure the voltage in a circuit?

With a voltmeter, you measure a voltage between two points in a circuit...

Circuit diagram with voltmeter for voltage measurement.

Circuit with voltmeter for voltage measurement. (Enlarge)

For measuring voltage you add the voltmeter using two nodes in the circuit. The to be measured voltage applies now to the voltmeter. This is a direct consequence of the Kirchhoff's Voltage Law (KVL). Now, you read the value of the voltage in Volt [V] at the voltmeter.

But pay attention to two points...

• To insert the voltmeter, the circuit needs not to be opened up!
• In an ideal measurement the voltmeter does not affect the result...

                 

Ideal Voltmeter An ideal voltmeter has an infinite high internal resistance (Ri = ∞ Ω). No additional current flows through the voltmeter and therefore has no influence on the measurement result.

An ideal voltmeter is unrealistic. But what is a real voltmeter and what influence does it have...?