Our instrument is the voltmeter and allows us to measure the difference in potentials in the same way that we can see the difference in water levels from this view of the glass and clear plastic containers.



If we put a ruler against the side view we can measure from the bottom of each tube to the water level in that tube.

This is the same as puting a voltmeter between two potentials.

If this were a line of dry cell batteries we would simply measure the voltage of each battery by puting the voltmeter between the poles.

In some cases this would give us the relationship between the batteries, as we would be able judge the driving force EMF of each against the driving force of the others.

This is similar to the laboratory procedures for measuring corrosion reactions.