Many of the freshly gathered cells of Valonia ventricosa have a resistance to direct current which is variable and depends on the potential applied. It is low when low potentials are applied and rises sharply at higher values. The rise may be more than 100 per cent in the cell as a whole, which is equivalent to several hundred per cent in the protoplasm alone. The rise becomes less as the cells stand in the laboratory, until a maximum is reached at all applied potentials, low and high, below the breakdown value (about 100 mv.): the cells are then said to be in a constant state.
During the variable state, the resistance rises when the positive current enters the protoplasm from outside, and falls when it passes out from the vacuole (this is determined by killing one end with chloroform).
The rise of resistance becomes faster with closely succeeding applications of potential. This is ascribed to the removal from the protoplasm of ions to which it is reversible. There is some evidence that these may be potassium ions.
Much of the apparent resistance rise may be accounted for by a back E.M.F. caused by the flow of current.