First off we had the Onion. The Onion gave off around 1.2 Volts.

Next there was the Orange. It gave off 1.2 Volts which is basically the same as the onion.

There is also the potato. the potato gave off 1.4 Volts which surprised me because I thought that the whole way voltage worked was off of the acidity in it.

lastly and with the best outcome there is the lemon, it had 1.5 volts and just barely had more than the potato.

The way that the voltage works is the acidity of the fruit juice. You also can get even more voltage by stacking up the amount of fruit into the circuit but you have to do it in a special way to make sure the circuit is still complete