Robin2:
I would not use the word "ground". Ground has a special meaning related to the Atmega's power supply.In my earlier post I just said that it can measure the voltage difference between two analog pins.
If you don't believe me read the datasheet and try it. Connect a low voltage battery between the appropriate analog pins without making ANY connection between that battery and the Arduino GND. I learned about it from this Youtube video and I have tried it.
...R
I believe you. The appropriate analog pins let you measure electrical potential between them.
That's not the same as feeding power to one analog pin without connecting ground between the board and the power source.