How does the loading effect of a voltmeter in a high-rsistance circuit compare to the loading effect in a low-resistance circult?

Answer 1

The voltmeter has an internal resistance, which should be as high as possible. As this resistance draws current from the circuit under test, it will affect circuit operation. This is more pronounced in a high impedance circuit because the current drawn flows through higher resistances.

Answer 2

A voltmeter will have more impact in a high-resistance circuit than it will in a low resistance circuit. Even though the voltmeter has high resistance, it is not unlimited, even the digital varieties that have 10 or 20 megohms of input resistance. This needs to be accounted for when making measurements of high resistance circuits. Sometimes, you can even bias the results enough to disturb the operation of the circuit.

Take the case of using a 10 megohm meter to measure the voltage across the bottom resistor in a pair of 1 megohm resistors used as a voltage divider. If the meter had no effect, the meter would read 50%, because this is a 50:50 divider. But now, we introduce an extra 10 megohms on the bottom, making the bottom be 909090 ohms and changing the ratio from 50:50 to 47:53. That's a big shift.