Ix] EXCITED RADIO-ACTIVITY 281 
excited activity is confined to the cathode in a strong electric field. 
In weaker fields the activity is divided between the cathode and 
the walls of the vessel. This was tested in an apparatus’ shown in 
Fig. 53. 
Electrometer Battery 



Fig. 53. 
A is a cylindrical vessel of 5°5 cms. diameter, B the negative 
electrode passing through insulating ends C, D. For a potential 
difference of 50 volts, most of the excited activity was deposited 
on the electrode B. For about 3 volts, half of the total excited 
activity was produced on the rod 5, and half on the walls of the 
vessel. Whatever the voltage applied, the sum of the activities 
on the central rod and the walls of the cylinder was found to 
be a constant when a steady state was reached. 
When no voltage was applied, diffusion alone was operative, 
and in that case about 13 per cent. of the total activity was on the 
rod B. The application of an electric field has thus no influence 
on the sum total of excited activity, but merely controls the pro- 
portion concentrated on the negative electrode. 
A more detailed examination of the variation with strength of 
field of the amount on the negative electrode was made in a similar 
manner by F. Henning’. He found that in a strong electric field 
the amount of excited activity was practically independent of the 
diameter of the rod B, although the diameter varied between 
59mm. and 60 mms. With a small voltage, the amount on the 
negative electrode varied with its diameter. The curves showing 
the relation between the amount of excited activity and voltage 
are very similar in character to those obtained for the variation of 
the current through an ionized gas with the voltage applied. 
The amount of excited activity reaches a maximum when all 
the emanation X is removed from the gas as rapidly as it is 
1 Rutherford, Phil. Mag. Feb. 1900. 
2 Drude’s Annal. p. 562, 1902. 
