The paper is the first of a series dealing experimentally with various aspects of the conductivity of oxide cathodes. The present Part starts with a brief survey of existing knowledge and then proceeds to examine the practical circumstances under which a non-uniformity of potential gradient can arise across the matrix under a modest continuous current loading. Using active-nickel cores to enclose a rectangular slab of barium-strontium oxide, it is found by probe experiments that nonuniformity of potential gradient occurs only in the presence of residual gas, and that the gradient tends to increase in those parts of the matrix likely to accumulate negative ions.