dhenry:
2 x 10 Megohm resistors across it (R1 and R2), making a 1:1 divider
That type of dividers are basically unusable for an avr's adc.
It was a hypothetical example to show the effect of a test instrument on the circuit being tested, in as simple terms as possible. The values were simply comparable to typical resistance of a voltmeter and values where any calculations were either simple enough to do in your head, or just intuitive to anyone with a little experience.