Why can a voltmeter potentially load a circuit?
Let's say the internal impedance of the voltmeter is 1 MOhm. So, if you have a pretty high resistance circuit, and you are measuring the drop across say, a 1MOhm resistor, then you make the effective resistance 0.5 MOhm - that's a non-negligible effect.
You want the internal impedance of the voltmeter to be large compared to the effective resistance across the points that you're measuring the voltage drop.
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