The Influence of Surface Conditions and Space Charges on the Conductivity of Poor Conductors

Surface conditions.---To keep a substance non-conducting it is necessary not only that it does not itself generate carriers of electricity, but that these are prevented from entering if good conductors are brought into contact with it. Vacuum and crystals are insulators because the electrons from metal electrodes cannot leave the metal on account of the work function. Water has electrolytic, not electronic conductivity, because ions can be discharged at the electrodes, while electrons cannot permeate the surface directly.Space charge.---The possible types of space charge in a medium containing ions of constant and equal mobility are discussed. It is pointed out that for the potentials which are of interest diffusion plays a role only in the immediate neighborhood of the electrode, and space charge is entirely due to insufficient recombination. Methods of determining the data of interest from space charge curves are given. The number of ions present is always below the equilibrium number.