Vol. 7, 1921 
PHYSICS: DUANE, PALMER AND YEH 
239 
In making a measurement of h, one experimenter observes the galvanom- 
eter attached to the potentiometer, and by varying a resistance in series 
with the X-ray tube keeps the difference of potential applied to it constant 
during the experiment. A second observer measures the current in the 
spectrometer's ionization chamber. A series of readings is taken on the 
two sides of the spectrometer's zero near the points at which the continuous 
X-ray spectrum vanishes. Curves A and B in the figure represent the 
ionization currents as functions of the angles that fix the positions of the 
reflecting crystal in one of the experiments. The horizontal portions of the 
curves correspond to the currents due to natural leak and to stray radiation. 
GG° 75° 79° 
Crystal Table Angle 
The inclined portions represent the increase in these currents due to the 
continuous X-ray spectrum. The readings corresponding to the short 
wave-length limit of the spectrum can be determined to within a few seconds 
of arc. As indicated in the figure, the difference between these readings 
on the two sides of the zero gives us twice the glancing angle 0, which 
must be substituted in the equation 
X = 2d sin 6 = 6.056 X sin 0 X 10- 8 cm. (2) 
in order to calculate the shortest wave-length X, in the continuous X-ray 
spectrum. 
A small correction has to be added to the observed value of 0, due to 
the fact that the source of rays and the slit of the spectrometer are not 
mathematical lines. The correction for the breadths of the source and 
slit we determine in two ways. Firstly, we estimate the apparent breadth 
