216 
MR. A. Ll. hughes ON THE 
Table II.— Cadmium. 
Wave-leng 
th. 
Velocities (in volts). 
Mean velocities. 
X 2537 
-901 
-901 
-890 
-897 
X2257 
1-420 
1-431 
1-431 
1-427 
X1849 
2-480 
• 2-480 
2-480 
2-480 
Wavelength. 
Frequency, 
11. 
Experimental 
velocities. 
V = hn - Vo. 
VV = h'n - c. 
X1849 
1623 X 1012 
2 - 480 volts 
[2-480] 
[2-480] 
X 2257 
1329 
1-427 
1-424 
1-339 
X2537 
1182 
-897 
[-897] 
[-897] 
X2967 
1010 
-148 
-286 
-495 
X3126 
960 
0 
-101 
-398 
X3340 
898 
No efiect. 
(--12) 
-293 
Vo = 3-347 volts; k = 3-590 x IQ-U 
accelerating potential of ’5 volt. Tliere was no leak due to X 3340 under the same 
conditions. In this region, perhaps the best test of the energy law is that there is an 
emission of electrons with X 3126, hut not with X 3340, and this is exactly what theory 
predicts. According to Ladenburg’s law, one might expect an emission of electrons 
down to X 4000. 
Dry oxygen at 140 mm. was admitted to the apparatus for 15 minutes. This caused 
a reduction (after evacuating again) in the total leak for each wave-length of about 
five times. The result of this was that it was impossible to measure the maximum 
emission velocity corresponding to X 1849. The velocity changes are :— 
X 1849 
X2257 
X2537 
effect too small. 
'276 volt reduction. 
•244 
5 ? 
5 9 
X 2967 no effect. 
The results of another experiment with a fresh surface of distilled Cd are given in 
Table III. 
As before, there is close agreement between the experimental value for X 2257 and 
the value predicted on the energy law. The energy law also predicts that the effect 
should set in between X 3126 and X 3340. 
The apparatus was filled with oxygen at 160 mm. for 10 minutes. A reduction in 
the total effect was again noticed. The velocity changes are :— 
X 1849 effect too small. 
X2257 '213 volt reduction. 
X2537 '244 „ 
X 2967 no effect. 
99 
