Contact Potential Measurements on Tungsten Filaments

Changes in contact potential of the surface of a tungsten filament have been measured by using a tube containing two filaments. Changes were produced by varying the amount of thorium on the surface, and by varying the temperature. (a) Changes produced by activation. When electron emission ($i$) with very low accelerating fields is compared with the contact potential ($V$) measured on the same surface and at the same temperature, the theoretical law $\frac{\mathrm{dlni}}{\mathrm{dV}}=\frac{e}{\mathrm{KT}}$ is obeyed. If emission under influence of higher fields, or contact potential at temperatures different from that of emission are used for comparison agreement does not exist. (b) Changes produced by temperature variations. Both activated and deactivated thoriated tungsten surfaces showed an increasingly negative contact potential (increasing work function) with rising temperature, the activated surface having the larger rate of change. The difference between the temperature coefficients of the activated and deactivated surfaces observed was 3.3×${10}^{-4\mathrm{}}$ volt/deg. A relatively large anomalous effect observed at temperatures below the emission range is ascribed to a reversible gas reaction. Pure tungsten seems to show a temperature coefficient of opposite sign, indicating a decreasing work function with rising temperature.