188. Effect of variation of E.M.F. on amount of excited activity from thorium. It has been shown that the 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 apparatus18 shown in Fig. 70.
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 B, 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 proportion 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. Henning19. 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 ·59 mm. and 6·0 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 active matter is removed from the gas as rapidly as it is formed. With weaker fields, a portion diffuses to the sides of the vessel, and produces excited activity on the positive electrode.
189. Effect of pressure on distribution of excited activity. In a strong electric field, the amount of excited activity produced on the cathode is independent of the pressure down to a pressure of about 10 mms. of mercury. In some experiments made by the writer20, the emanating thorium compound was placed inside a closed cylinder about 4 cms. in diameter, through which passed an insulated central rod. The central rod was connected to the negative pole of a battery of 50 volts. When the pressure was reduced below 10 mms. of mercury, the amount of excited activity produced on the negative electrode diminished, and was a very small fraction of its original value at a pressure of ⅒ mm. Some excited activity was in this case found to be distributed over the interior surface of the cylinder. It may thus be concluded that at low pressures the excited activity appears on both anode and cathode, even in a strong electric field. The probable explanation of this effect is given in the next section.