Confirmation of Crystal Wave-Length Measurements and Determination ofhe43

By observation of x-ray continuous spectrum isochromats the potential necessary for the production of general radiation capable of reflection from calcite at a known Bragg angle $\theta$ has been determined. By using a double spectrometer and tungsten source, and reflecting at the Bragg angle for Ag $K{\alpha }_1$ the value $V^{\prime }$ $sin\theta =2035.3\mathrm{}\pm 0.2$ international volts has been obtained. This is very close to Feder's value, 2036.0. In agreement with Feder it is concluded that Wagner's "low" value is in error because of experimental conditions and that this error can not now be rectified. It is shown that the "high" value of Duane, Palmer and Yeh is subject to certain calculable corrections which lower it to 2034.6, in much closer agreement with present results. The wave-length of Ag $K{\alpha }_1$ as given by the law of Duane and Hunt is found to be in agreement with the value obtained from crystal measurements and at variance with ruled grating results. Utilizing crystal wave-length measurements by Bragg's law the present results yield $\frac{h}{e^{\frac{4}{3}}}=(1.7559\mathrm{}\pm 0.0002)\times {10}^{-14\mathrm{}}$ if Birge's (1929) auxiliary constants are employed, and 1.7563 with more recently determined values of five of these constants. Assuming the 1930 oil drop value $e=(4.770\mathrm{}\pm 0.005)\times {10}^{-10\mathrm{}}$ e.s.u. this investigation gives $h=(6.546\mathrm{}\pm 0.006)\times {10}^{-27\mathrm{}}$ erg·sec.