This third container is put in the large plastic container on a glass stand so that the bottom is not level with the bottoms of the other small containers.



This means that you cannot assess the current flow in this container as it bears no relationship to the possible currents flowing in any of the other small containers.

This is similar to an active corrosion cell on a pipeline as the corrosion current flowing in this cell is independant of the current flowing in any of the other corrosion cells on the surface of that pipeline.

The difference in water levels can be likened to the difference in potentials. If we could measure the difference in water levels in the containers, we still would not know the level of the bottom of the containers.

We have no zero potential against which to measure the corrosion potential.