Page:Radio-activity.djvu/267

 closed vessels the proportion of the total current, due to the emanation, varies with the distance between the plates as well as with the thickness of the layer of active material. It also varies greatly with the compound examined. In the nitrate, which gives off only a small amount of emanation, the proportion is very much smaller than in the hydroxide, which gives off a large amount of emanation.

143. Increase of current with time. The current due to the emanation does not reach its final value for some time after the active matter has been introduced into the closed vessel. The variation with time is shown in the following table. The saturation current due to thorium oxide, covered with paper, was observed between concentric cylinders of 5·5 cms. and ·8 cm. diameter.

Immediately before observations on the current were made, a rapid stream of air was blown through the apparatus. This removed most of the emanation. However, the current due to the ionization of the gas by the emanation, as it was carried along by the current of air, was still appreciable. The current consequently does not start from zero.

Time in seconds Current 0           9      23           25      53           49      96           67     125           76     194           88     244           98     304           99     484          100

The results are shown graphically in Fig. 52, curve B. The decay of the activity of the emanation with time, and the rate of increase of the activity due to the emanation in a closed space, are connected in the same way as the decay and recovery curves of Th X and Ur X.

With the previous notation, the decay curve is given by

I_{t}/I_{0} = e^{-λt},

and the recovery curve by

I_{t}/I_{0} = 1 - e^{-λt},

where λ is the radio-active constant of the emanation.

This relation is to be expected, since the decay and recovery