One area that may be of interest to readers is the matter of how ionic current flow in solution differs from electron current flow in solid conductors. Electrons have tiny mass and size, and move through metal conductors not continuously, but by dislodging nearby electrons in the outer orbits of metallic atoms, in a billiard-ball effect that operates with little resistance and at tremendous speed. The resistance to the passage of this cascade of electron current is so small that it normally offers no great source of error in measuring a resistance value some distance away from the driving source for the current.

Ions are quite another matter. Compared to electrons they are huge in both mass and physical size. In a properly designed conductivity measurement, there is no electron passage form the electrodes into or out of the stream. Rather, the surface of the electrodes charge like capacitors, with excess electrons piling up at the surface of one electrode, and a corresponding shortage of them created at the other. The voltage created by these localized charge concentrations attracts oppositely charged ions in the solution, which move far more slowly to pile up against the outer surfaces of the electrodes, not quit touching but very close. These great cumbersome particles must move their bulk and weight through the viscous medium of the solvent and all its molecules. Further impediment to their motion is added by the fact that when passing close to oppositely charged ions moving in the other direction, the electrical attraction between the two will interfere with the linear forward motion of both. Finally, as if the ions themselves weren’t already large enough, many of them exist in a hydrated state in solution. That is, they attract and retain a certain number of water molecules in close proximity to themselves because water molecules are electrically polarized, having a predominately negative potential at one end and positive at the other. Thus, the negative end of a certain number of them will be attracted to positive ions and vice-versa. How many depends on the structure of the attracting ion. So now the ion has to carry baggage that can easily exceed its own physical size, making it even more difficult to pass through the cloud of mixed molecules that is the electrolyte solution. Imagine yourself trying to fight across a packed terminal to an airline gate carrying half-dozen suitcases, sidetracking toward people who you recognize as they pass by. All these factors, and some more esoteric ones not mentioned, combine to give dissolved electrolytes unique and widely varying current carrying ability in response to voltage stimulus from the conductivity sensor electrodes. Finishing the airline analogy, consider that you’d have to revers your direction of travel every ten milliseconds or so.