Your son's results

the following results (fruit/pH/mV): lemon/2.22/997; grapefruit/2.95/923; pineapple/3.08/997; kiwi/3.45/663; pear/3.74/770; apple/3.80/1038; onion/4.04/741; tomato/4.15/955; peach/4.42/909; banana/4.82/892; and cantaloupe/6.24/873.

indeed indicate a weak or non-existent correlation between pH and conductivity.

To help make sense of this (because in cases like this it's better to point to where people can find the answer rather than just tell them) I will tell you a few things:

• Conductivity/resistance and voltage are not the same thing.
• Voltage in a battery is largely dependent on the composition of the electrodes. Using, say, gold instead of copper would give you different voltages!
• You might want to check the effect of the distance between your electrodes on both conductivity and voltage.
• Conductivity (resistance) is largely dependent on the total ion concentration between the electrodes.
• Think of an experiment in which you can compare conductivity with a precisely known total ion concentration. (Hint: salt water!)
• What difference in concentration of hydronium ions does a pH range of 2.22 to 6.24 correspond to? Is this difference likely to be significant? Why or why not? (Think of the concentration of other ions in your fruits.)

I hope this helps you understand a bit more of what is going on in what is, after all, a rather difficult experiment!

Dan Berger