Comparison of Resistance Changes in Collodion Membranes and Cells.

Most conductors, both metallic and electrolytic, follow Ohm's law, the current being proportional to the applied potential, and the resistance a constant (with exceptions in the case of temperature rise or dielectric breakdown). In the large cells of Valonia and Nitella, however, the resistance becomes higher with increase of potential from 25 to 150 millivolts, under some conditions. 1

This discovery led to a search for a non-living model of this phenomenon and such has been found in dry collodion films, thinner than those used by Michaelis, 2 and Northrop; 3 thicker membranes show the effect in smaller degree. When direct current is passed across such thin films separating 2 solutions of good conductivity, the resistance is found to vary with the applied potential. With sea water on both sides, the resistance is a minimum below 100 millivolts, and may rise as much as 100% when the potential is increased to 3 or 4 volts. Above this voltage irregular results are obtained.

The course of the resistance rise is very regular and reproducible, resembling the exponential curve of a condenser charging-current. There is, however, no E.M.F. of polarization, detected on breaking the current, which could account for more than a few per cent of the increase of resistance. The lower value is regained apparently as soon as the return to a lower measuring potential is made; although some effect persists, since the second and third applications of high potential in quick succession cause more rapid rise of resistance than the first.

When the membrane separates 2 solutions of the same concentration but of different conductivity, such as 0.5 N KC1 and HC1, the direction of the current is found to influence the resistance change. Thus when hydrogen ion is moved across the membrane, the resistance falls slightly with increasing potential, while when potassium ions are caused to pass by reversing the current, the resistance rises as much as 100% in changing from 0.1 to 3.0 volts.