Page:Radio-activity.djvu/338

 The effect of heat on the volatilization of the active deposit of radium has been examined in detail by Curie and Danne. The interesting and important results obtained by them will be discussed in chapter, section 226.

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 apparatus shown in Fig. 70.

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. 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