This circuit consists of a power source with a max of 9VDC and a voltage divider, which steps down the voltage to a max of 5VDC, which is also the max input for the most Arduino boards.
The analog read function will return a reading from 0-1023 (0V-5V), this can still be done since the voltage divider also scales down the voltage (at 4.5VDC the arduino receives 2.5VDC)
Some further functions combined will now allow you to measure not only 0-5VDC with an arduino but basically any DC voltage you'd like, as long as you can step the voltage down to 5VDC max and limit the current to <20mA
Have fun!
