A Precision Evaluation ofheby X-Rays

Precision x-ray measurements of $\frac{h}{e}$ have been made at potentials from 8000 to 19000 volts. A double crystal spectrometer with perfect calcite crystals was used as the monochromator. The window width was 0.7 volt. A unipotential cathode was developed and the use of this with the narrow monochromator window gave isochromats which broke very sharply at the high frequency limit. Isochromats were recorded with both tungsten and oxide cathodes which experimentally indicated the necessity for making the work function correction to the applied voltage. No evidence was found to support the previously reported effects due to surface films on the target or gas pressure in the x-ray tube. Targets of Au, W, Ta, Mo, Ni, and Cu were used. The pronounced structure near the high frequency limit appears to be much the same for all targets, differing only in some details and magnitude. The value found for $\frac{h}{e}$ is 1.37938±0.00008×${10}^{-17\mathrm{}}$ erg·sec.·${\mathrm{e}.\mathrm{s}.\mathrm{u}.\mathrm{}}^{-1\mathrm{}}$ and is independent of the target or voltage used.