Voi,. 7, 1921 
PHYSICS: DUANE, PALMER AND YEH 
241 
is about 20° Centigrade. The value of V as measured by the potentiom- 
eter is 24,413 volts in each case. We estimate that the error of precision 
in the product V sin 0 amounts to about one part in two thousand. 
Column 6 contains the values of h calculated from the values of VsinO. 
Since several constants occur in these calculations (the velocity of light, 
the grating constant of calcite and the charge on the electron), the values 
of h are not quite as accurate as those of Vsin 6. We estimate the probable 
error in h to be about fifteen parts in ten thousand. This gives us an 
average 
h = (6.556 ± .009) X 10" 27 . 
This value of h agrees with that previously published by Blake and 
Duane. 3 It is, however, a fraction of one per cent larger than the value 
recently obtained by E. Wagner in a careful series of measurements. 
In the experiments described above the X-rays left the target in a 
direction at right angles to the line of motion of the cathode particles. An 
interesting question has been raised recently as to whether the limit 
of the continuous spectrum would be altered if the rays came off at some 
other angle. 4 To test this point with the accurate method of measuring 
the voltage which we now have, we made a series of experiments with an 
ordinary Coolidge tube (tungsten target) placed so that the X-rays that 
passed through the spectrometer's slit left the target at an acute angle 
of about 45° to the direction of the cathode stream. The results of this 
series of measurements appear in table 2. As in the previous experi- 
ments the voltage applied to the tube amounted to 24,413 volts. The 
X-ray tube had no thin mica window, so that the accuracy appears to 
be somewhat less than in the previous series of measurements. The 
value of Vsin 6, however, does not differ from that for rays at right 
TABLE 2 
Glancing Angles and Radiation Constant 
DATE 
9 (uncorrected) 
Ad 
0 (CORRECTED) 
VsinO 
h X 10-" 
April 6 
4°-46'-35" 
V 
4°-47 , -35" 
2039.8 
6.556 
April 6 
4°-46'-33" 
V 
4°-47 , -33" 
2039.6 
6.555 
April 12 
4°-46'-53" 
48" 
4°_47'HL1" 
2040.7 
6.558 
April 27 
4°-46 / -43' / 
47" 
4°-47 , -30" 
2039.3 
6.554 
2039.9 6.556 
angles to the cathode stream. There does not appear to be a Doppler 
effect for the short wave-length limit of the spectrum that amounts to as 
much as one part in two thousand. This agrees with Wagner's results. 4 
A very much more detailed report of this research is being published in 
one of the Physical Journals. 
1 Duane and Hunt, Physic. Rev., Ithaca, Aug., 1915, p. 166. 
2 See a report in the Jahrbuch der Radioahctivitat, etc., for 1919 by Wagner, and 
