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Kirchhoff's Law » Physics » Electricity » Measuring Current-Voltage » Let's go... » Experiment 5 - Real Voltmeter

Experiment 3 - The Real Voltmeter


Real Voltmeter

The model of a real voltmeter consists of an ideal voltmeter  and an resistor Ri in parallel.


Equivalent circuit diagram real voltmeter.


Circuit diagram with a real voltmeter.

In the illustrated diagram, this has no influence on the measurement. But only as long as you assume an ideal battery/power supply.

In the circuit, an additional current flows through the internal resistance Ri...

  • With a real voltmeter a larger current flows through the circuit, as in a circuit without voltmeter!

This results in a higher load, and with a real battery results in a lower measured voltage.

Typical values of the internal resistance are serveral several MΩ for almost all digital multimeters and analog multimeters with a preconnected amplifier. For analog multimeters without amplifier, the internal resistance is often as low as a few few kΩ.

You can examine this by connecting the analog and digital voltmeter and measuring the voltage at different measuring ranges. Thereby you see that the measured values differ due to the internal resistance.

In the next experiment you consider the consequences if you measure current and voltage...

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