initial value, and afterwards decayed at the normal rate. The dotted line curve D represents the variation of activity to be expected if the activity decayed exponentially with the time. The explanation of this remarkable action is considered in detail in section 207.
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Fig. 65.
181. Effect of dust on the distribution of excited activity.
Miss Brooks[1], working in the Cavendish Laboratory, observed that
the excited activity due to the thorium emanation appeared in
some cases on the anode in an electric field, and that the distribution
of excited activity varied in an apparently capricious manner.
This effect was finally traced to the presence of dust in the air of
the emanation vessel. For example, with an exposure of 5 minutes
the amount of excited activity to be observed on a rod depended
on the time that the air had been allowed to remain undisturbed
in the emanation vessel beforehand. The effect increased with the
time of standing, and was a maximum after about 18 hours. The
amount of excited activity obtained on the rod was then about
20 times as great as the amount observed for air freshly introduced.
- ↑ Miss Brooks, Phil. Mag. Sept. 1904.
