The Contact Difference of Potential Between Tungsten and Barium. The External Work Function of Barium

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Abstract
An intensive study of the contact difference of potential between tungsten and barium has been made with the purpose of developing a generally applicable method of measuring contact potential differences under the most rigorous conditions possible. Extreme outgassing of surfaces in the highest attainable vacuum is found to be essential but when these requirements are met highly reproducible measurements can be obtained. The tubes are sealed from the pumps after baking, gettered with barium, and immersed in liquid air during measurement. The tungsten, in the form of a thin ribbon, is cleaned by flashing at 2800°K until its work function remains unaltered by further heating. Barium films are deposited on the tungsten ribbon by thermal vaporization and their reproducibility checked by measuring a succession of films formed in a fractional distillation of the metal. The measuring technique is designed to detect any changes in work function which may occur immediately after the preparation of a clean surface and to prevent accidental contamination of those parts of the tube which must be maintained at constant work function. Sharp localization of the surface areas for which electron current-potential characteristics are taken and an increased sensitivity of measurement are secured by using a narrow and intense beam of electrons and determining displacements of the characteristics in the retarding potential region. Measurements with three types of tubes and with samples of tungsten and barium from several different sources give the value 2.13±0.05 volts for the contact difference of potential between tungsten and barium; 2.39±0.05 equivalent volts for the external work function of barium. The method can, presumably, be quite generally applied to the measurement of the contact potential difference between a tungsten reference surface and any other metal and is adapted, also, to the study of dilute films of any metal on tungsten.