UNDER THE INFLUENCE OF CHEMICAL ACTION. 
27 
light in fig. 13. According to measurements of the writer and K. T. Compton,* curves 
plotted from such data intersect the frequency axis at the threshold frequency v 0 . If 
we assume the half-millimeter deflection with X 4916 to mean something, this fixes the 
position of v 0 at the point A or 6-02 X 10 14 . On the other hand, if the reading at 
X 4916 is disregarded, it follows from a consideration of the X 4055 and X 4355 points 
alone that the intersection cannot lie to the right of the point B or 6 -25 X 10 14 , which 
is the point where a straight line through these two points meets the axis. Thus this 
experiment establishes the value of v 0 as lying between the limits 6-02 X 10 14 and 
6 -25 X 10 14 . It is unfortunate that the line X 4916 is so weak, otherwise these limits 
could be narrowed very considerably. 
Immediately after determining v 0 the measurements for determining the intersection 
of the photo-electric characteristic for the line violet 2 (X 4046 to 4077) with the voltage 
axis were taken. The pressure was still 0 -0045 mm. The photo-electric currents, in 
scale divisions in 60 seconds with 0 -01 mfd. additional capacity connected with the 
electrometer, were as shown in the following table in the order in which the points 
were taken:— 
Table VI. 
Volts . . . 
- 3 
— 1-8 
- 1-0 
— 1-2 
— 1-1 
— 1-3 
Current . 
112 
101 
0-5 
12 
3 
29-5 
These data are plotted in fig. 14, from which it will be seen that the photo-electric 
characteristic for frequency 7 -40 X 10 14 cuts the voltage axis at — 1 -07 volts. Thus 
the true zero on the volt scale at this time was — (1 -07 — h/e (v — i/ 0 )), where 
v — 7 -40 X 10 14 and h/e, — 4 -124 X 1(T 15 , is expressed in appropriate units to give 
volts. Now h/e (v — v 0 ) is equal to -569 or -474 volt, according to which of the 
limiting values of v 0 determined above we take. Adding 1-07 to these, it follows that 
* ‘ Phil. Mag.,’ vol. 26, p. 562 (1913). 
E 2 
