Page:Radio-activity.djvu/409

 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.

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 * 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

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 * Time in | Calculated value | Observed value |
 * minutes |  of activity    |  of activity   |
 * 0   |      100         |     100        |
 * 10   |       96·8       |      97·0      |
 * 20   |       89·4       |      89·2      |
 * 30   |       78·6       |      80·8      |
 * 40   |       69·2       |      71·2      |
 * 50   |       59·9       |      60·8      |
 * 60   |       49·2       |      50·1      |
 * 80   |       34·2       |      34·8      |
 * 100   |       22·7       |      23·2      |
 * 120   |       14·9       |      15·2      |

exposure (see Fig. 88). This is shown by the agreement of the numbers in the above table. The first column in the table