Voltmeter

An ideal voltmeter has infinite resistance: It is an open circuit. Although it is impossible to make a physical voltmeter with infinite resistance, a well designed voltmeter exhibits a very large internal input resistance. In some experiments, it is important to take into account the finite, non-ideal, internal resistance. To determine the internal resistance of the voltmeter, set up the circuit shown in figure . The voltmeter reads the voltage across itself, which includes its internal resistance. Since the circuit has only a single branch, the current flowing through the resistor also flows through the voltmeter. The current is given by the equation:

From Ohm's Law, if we know the current (I) and the voltage () we can compute .

  
Figure: Circuit for measuring the resistance of the voltmeter.

1. Select a 1 resistor.
2. Measure its value using the multimeter.
3. Set the power supply to provide 10 V (Remember, always measure the voltage provided by the power supply with either the voltmeter or the scope. Do not rely on the digital display on the front panel of the power supply.)
4. Assemble the circuit in Figure .
5. Record the voltage measured by the voltmeter
6. Compute the internal resistance of the voltmeter using Equation .