When you hook up a voltmeter to a circuit, you are providing another possible pathway for current—that is, through the voltmeter. Usually this is not a problem, provided that the resistance of the voltmeter is much larger than the resistance of the circuit. Suppose you have a circuit containing a DC supply (voltage V) and a resistor R, and you want to use a voltmeter (with resistance 10 MΩ or 107 Ω) to measure the voltage drop across a resistance R. What is the largest value of R for which the current drawn from the DC supply will change by no more than 1%? (Hint: the current through R alone is I=V/R, where V is the DC supply voltage. What is the current through the parallel combination of R and the voltmeter?)
The Attempt at a Solution
I really have no idea how to go about this. Only having one number really throws me for a loop. Right now I'm still trying to figure out what equations I need to use.