Vol. 8, 1922 
PHYSICS: CLARK AND DUANE 
93 
power delivered to the tube, constant. Evidently the ionization current 
will not completely vanish until the voltage becomes so low that it cannot 
produce the longest X-rays reflected at the angle d according to equation 
(1). The longest waves correspond to n = 1. 
A small correction has to be added to the angle 6 due to the fact that 
the source of rays and the slit of the spectrometer have finite breadths. 
The rays in the incident beam are not all exactly parallel to each other 
and the correction equals half the angle between the rays that 
deviate most from the central ray on its two sides. This angle is the 
breadth of the source plus the breadth of the slit divided by the distance 
between them. The correction is usually small. In our experiments it 
amounts to about 3 ' of arc, which is less than one per cent of the corrected 
value of 6 = 5°. 51 used in the first illustrative case described below. 
1 
1 
\ 
i 
44- 
7- 
C 
Y 
l 
A 
L 
U 
1 
PI 
Y 
d 
0 
1 
^$ 
)q 
«» 
cm 
i 
1<] 
jt 

h 
Ifl 
,t. 
2C 
< 
■f 
l.'i 
/I 
1 
11 
V 
J 
i 
[)? 

\- 
/ 
li 
0 
X 
! 
V 
/ 
} 
/ 
FIGURE 2 
The curves in figure 2 represent the ionization currents as functions of 
the voltages for three of the peaks of figure 1 . The voltages at which the 
ionization currents vanish for the peaks marked 100 and 110, respectively, 
are 18,600 and 26,310 volts. 
Eliminating X between equations (1) and (2') we get, with w = 1, 
, = (3) 
2Vsmd ^ ^ 
Substituting in this 6 = 5°. 51 and the values of V, we find for d the 
3.535 and 2.495 Angstroms, respectively. The ratio of the first to the 
