Part of the radio-active dust is also carried to the anode, and the proportion increases with the length of time during which the air has been undisturbed. The greatest amount obtained on the anode was about 60% of that on the cathode.
These anomalous effects were found to disappear if the air was made dust-free by passing through a plug of glass wool, or by application for some time of a strong electric field.
182. Decay of excited activity from radium. The excited activity produced on bodies by exposure to the radium emanation decays much more rapidly than the thorium excited activity. For short times of exposure7 to the emanation the decay curve is very irregular. This is shown in Fig. 66.
It was found that the intensity of the radiation measured by the α rays decreased rapidly for the first 10 minutes after removal, but about 15 minutes after removal reached a value which remained nearly constant for an interval of about 20 minutes. It then decayed to zero, finally following an exponential law, the intensity falling to half value in about 28 minutes. With longer times of exposure, the irregularities in the curve are not so marked.