One of the things that may not be "obvious" from the experiment we did is that redox reactions which can be used to generate voltage are not as restricted as the procedure we used might imply. Almost any metals with almost any electrolytes (as opposed to a metal with its own ion in solution) will generate some kind of electric potential. In the case of elements which are not metals or even those which are in ionic form, electrical contact is often possible through inert conductors like graphite or platinum.
However, the most stable results are generally obtained with electrodes related to the ions in solution.
When a voltmeter is used to detect electric potential, certain conventions are used to determine the eventual direction of electron flow. We must say "eventual" because the resistance to electron flow in a voltmeter is generally very high and no actual reaction takes place when electrodes are connected to the voltmeter. If the electrodes are joined directly with a wire, electrons flow in the direction indicated by the voltmeter.