The electric current I flowing in a streaming cell if the electrodes, which are supposed to be ideally depolarized, are short-circuited. I is positive if the current in the membrane, plug, etc., is from high to low pressure side (and in the outside lead from low to high pressure side).
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 620 (https://doi.org/10.1351/pac197231040577)