The apparent conductivity of oxide coatings used on emitting filaments

1. Recent experiments by Reimann and Treloar among others have thrown further light on the way in which the electrical current supplying the filament emission passes through the coating of barium- or strontium-oxide, used to render the filament a low-temperature emitter. They have studied in particular the variation of the current flowing with applied potential and with temperature. Though these phenomena have been elaborately discussed already by these authors and also by Reimann and Murgoci, by Becker and by Lowry, it seems to us possible that something new can be said in the light of the more detailed theory of semi-conductors and rectifying contacts now available. It is possible that in return for the study of conductivity some light may be thrown on the most perplexing feature of thermionic emission from oxide-coated cathodes—its failure to saturate properly for moderate voltages. The recent experimental work of Reimann and Treloar need only be shortly summarised here. The conductivity measurements were made by measuring the current between two oxide-coated nickel filaments twisted tightly together and recoated after twisting. [The two twisted wires were used jointly as a single cathode for thermionic measurements.] The conductivity as measured is therefore derived from the current flowing from metal electrode to metal electrode for given potential difference between the electrodes, deeply immersed in the oxide and separated by a thickness of oxide of the order of 0⋅02 mm.