Free Barium and the Oxide Cathode

It is shown that the greater part of the free barium produced by a modern oxide-coated cathode is formed through chemical reduction by the impurities in the nickel core. The actual amount of free barium retained by the cathode may only be of the order of a tenth of that produced, the rest being evaporated on to other electrodes. There appears to be a lower limit to the rate of production at a given temperature, below which the emission cannot be maintained at a high level. The free barium concentration in a well-activated coating is of the order of 0.01% or 3 × 10 17 atoms/c.c.