An ideal voltage-measuring device has infinite resistance. But in practical cases, it is not possible. Here our aim is to demonstrate a non-ideal voltage measuring device has non-infinite resistance.
A voltmeter is a device to measure the potential difference between two points. It measures the potential difference between two points by connecting voltmeter terminals to those two points without breaking the wire.
A voltmeter is connected parallel to the location being measured. The current passing through the voltmeter must be zero for the exact measurement of voltage. So the resistance of an ideal voltmeter is infinite.
Iv-current through voltmeter
when R is infinite, Iv becomes zero.
In a practical case, there is no ideal voltmeter. Each volt meters have non-infinite resistance.
Consider the circuit.
Voltmeter measures potential difference across AB.
Effective resistance of circuit = R+RRv/(R+Rv)