The amount of excited activity produced on a body increases at first with the time, but reaches a maximum after an exposure of several days. An example of the results is given in the following table. In this experiment a rod was made the cathode in a closed vessel containing thoria. It was removed at intervals for the short time necessary to test its activity and then replaced.
| Time in hours | Current |
|---|---|
| 1·58 | 6·3 |
| 3·25 | 10·5 |
| 5·83 | 29 |
| 9·83 | 40 |
| 14·00 | 59 |
| 23·41 | 77 |
| 29·83 | 83 |
| 47·00 | 90 |
| 72·50 | 95 |
| 96·00 | 100 |
These results are shown graphically in Curve B, [Fig. 64]. It is seen that the decay and recovery curves may be represented approximately by the following equations.
For the decay curve A,
For the recovery curve B,
The two curves are thus complementary to one another; they are connected in the same way as the decay and recovery curves of Ur X, and are susceptible of a similar explanation.
The amount of excited radio-activity reaches a maximum value when the rate of supply of fresh radio-active particles balances the rate of change of those already deposited.
180. Excited radio-activity produced by a short exposure. The initial portion of the recovery curve B, [Fig. 64], is not accurately represented by the above equation. The activity for the first few hours increases more slowly than would be expected from the equation. This result, however, is completely explained in the light of later results. The writer[[273]] found that, for a short exposure of a body to the thorium emanation, the excited activity upon it after removal, instead of at once decaying at the normal rate, increased for several hours. In some cases the activity of the body increased to three or four times its original value in the course of a few hours and then decayed with the time at the normal rate.