Page:Radio-activity.djvu/264

 the gas, which is a measure of the activity of the emanation, diminishes according to an exponential law with the time like the activity of the products Ur X and Th X. The rate of decay is, however, much more rapid, the activity of the emanation decreasing to half value in about one minute. According to the view developed in section 136, this implies that half of the emanation particles have undergone change in one minute. After an interval of 10 minutes the current due to the emanation is very small, showing that practically all the emanation particles present have undergone change.

Fig. 52.

The rate of decay has been more accurately determined by Rossignol and Gimingham who found that the activity fell to half value in about 51 seconds. Bronson, using the steady deflection method described in section 69, found the corresponding time 54 seconds.

The decrease of the current with the time is an actual measure of the decrease of the activity of the emanation, and is not in any