taking the value of λ2 = 5·38 × 10-4 and λ3 = 4·13 × 10-4.
| Time in minutes | Calculated values | Observed values |
|---|---|---|
| 0 | 100 | 100 |
| 10 | 96·8 | 97·0 |
| 20 | 89·4 | 88·5 |
| 30 | 78·6 | 77·5 |
| 40 | 69·2 | 67·5 |
| 50 | 59·9 | 57·0 |
| 60 | 49·2 | 48·2 |
| 80 | 34·2 | 33·5 |
| 100 | 22·7 | 22·5 |
| 120 | 14·9 | 14·5 |
The second column gives the observed activity (measured by means of an electroscope) for a long exposure of 24 hours in the presence of the emanation.
In cases where a steady current of air is drawn over the active body, the observed values are slightly lower than the theoretical. This is probably due to a slight volatility of the product radium B at ordinary temperatures.
Fig. 90.
223. Analysis of the α-ray curves. The analysis of the decay curves of the excited activity of radium, measured by the α rays, will now be discussed. The following table shows the variation of the intensity of the radiation after a long exposure in the presence of the radium emanation. A platinum plate was made active by exposure for several days in a glass tube containing a large quantity of emanation. The active platinum after removal was placed on the lower of two parallel insulated lead plates, and a saturating electromotive force of 600 volts was applied. The ionization current was sufficiently large to be measured by means of a sensitive high-resistance galvanometer, and readings were taken as quickly as possible after removal of the platinum from the emanation vessel. The initial value of the current (taken as 100) was deduced by continuing the curves backwards to meet the vertical axis (see [Fig. 90]), and was found to be 3 × 10-8 ampere.
| Time in minutes | Current |
|---|---|
| 0 | 100 |
| 2 | 80 |
| 4 | 69·5 |
| 6 | 62·4 |
| 8 | 57·6 |
| 10 | 52·0 |
| 15 | 48·4 |
| 20 | 45·4 |
| 30 | 40·4 |
| 40 | 35·6 |
| 50 | 30·4 |
| 60 | 25·4 |
| 80 | 17·4 |
| 100 | 11·6 |
| 120 | 7·6 |
These results are shown graphically in the upper curve of [Fig. 90]. The initial rapid decrease is due to the decay of the activity of the matter A. If the slope of the curve is produced backwards from a time 20 minutes after removal, it cuts the vertical axis at about 50. The difference between the ordinates of the curves A + B + C and LL at any time is shown in the curve AA. The curve AA represents the activity at any time supplied by the change in radium A. The curve LL starting from the vertical axis is identical with the curve already considered, representing the decay of activity measured by the β rays for a long exposure (see [Fig. 88]).