In practice, to measure voltage you have to draw a small amount of current. So if you put a large resistor (say 1 Megohm) as your "R" and measured with a meter there would be a voltage drop, caused by the measuring current drawn by the meter.
In practice, to measure voltage you have to draw a small amount of current. So if you put a large resistor (say 1 Megohm) as your "R" and measured with a meter there would be a voltage drop, caused by the measuring current drawn by the meter.