EMISSION VELOCITIES OF PHOTO-ELECTRONS. 
213 
circles inside the apparatus. Thus a magnetic field would account for the part B/>, 
fig. 3, whicli is usually attributed entirely to reflection of electrons. 
From a consideration of these eftects, one must conclude that the velocity 
distribution curves usually obtained do not give anything like the actual distribution 
of velocities with which the electrons emerge. Many velocity distribution curves 
have been given and discussed in various papers, but the complications arising from 
effects (C) and (D) have been completely overlooked. 
Experimental .—Experiments were made to see how the velocity distribution curves 
could be affected by magnetic fields and the presence of gas. In particular, the 
corresponding effect on the maximum velocity was investigated. The apparatus used 
is shown in fig. 2. The disc N was covered with a layer of zinc. The unresolved 
light from a mercury arc was used. 
In column I. the results obtained under normal conditions are given. The 
vacuum was obtained by the liquid air method, and the pressure was certainly below 
’0005 mm. The earth’s magnetic field (about ’5 gauss) was approximately parallel 
to the disc. 
In column II. the results are given when an electromagnet was held near the 
apparatus. The field inside the apparatus was far from uniform, but at the centre 
it was about 7 gauss. 
In column III. the results are given when the magnetic field was increased to 
30 gauss. 
The results given in column IV. were obtained with air in the apparatus at a 
pressure of ’03 mm. 
The results given in Table I. are plotted in fig. 7. 
Table I. 
Potential. 
Photo-electric current. 
1 . 
11 . 
III. 
IV. 
40 volts 
83 
8G 
30 
83 
85 
20 
82 
80 
10 
82 
82 
72 
79 
4 
82 
80 
54 
71 
2 
82 
77 
38 
63 
1 
80 
72 
19 
52 
0 
35-5 
22-5 
3-5 
18 
- -7 
17-2 
7-0 
1-0 
5 
- 1-0 
10-0 
4-5 
•5 
2-5 
- 1-3 
5-0 
2-0 
0 
•5 
- 2-0 
0 
0 
0 
0 
- 10-0 
-1-5 
- 1-0 
0 
0 
Maximum velocity . . . 
1 • 93 volts 
1-76 volts 
1 • 86 volts 
