Soft X-Rays: Improvements in Technique and New Results for C, Cu, and W

Improved technique in soft x-ray measurements.—A new tube designed for high sensitivity in detecting soft x-rays by their photoelectric action and using a system of gauzes to exclude ions from the detecting plate was found to give results identical with the previous tube in which a vane system was employed. In either type of tube the range of voltage applied to exclude ions from the detecting plate must be determined. Failure to observe this seems to account for some results which have appeared in the literature of this subject. Greatly increased precision was secured by measuring both the thermionic and the photoelectric currents, $I$ and $E$, by balanced methods in which the full scale of the instruments was used to measure current increments, instead of total currents, as the voltage was increased in small steps. The applied voltage $V$ was measured by a similar balanced method in which residuals only were read on the voltmeter. By these devices the observations were so precise that critical potentials could be determined from curves in which were plotted the second differences of the ratio $\frac{E}{I}$ against $V$, which rendered discontinuities much more marked than in previous methods. The method is illustrated by application to tungsten in a small voltage range.