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 the above equation. Some of these cases will be considered in section 47.

27. The general shape of the current-voltage curves for gases exposed to the radiations from active bodies is shown in Fig. 3.

This curve was obtained for ·45 grams of impure radium chloride, of activity 1000 times that of uranium, spread over an area of 33 sq. cms. on the lower of two large parallel plates, 4·5 cms. apart. The maximum value of the current observed, which is taken as 100, was 1·2 × 10-8 amperes, the current for low voltages was nearly proportional to the voltage, and about 600 volts between the plates was required to ensure approximate saturation.

In dealing with slightly active bodies like uranium or thorium, approximate saturation is obtained for much lower voltages. Tables I. and II. show the results for the current between two parallel plates distant 0·5 cms. and 2·5 cms. apart respectively, when one plate was covered with a thin uniform layer of uranium oxide.

0·5 cms. apart

Volts  Current ·125 18   ·25   36   ·5    55  1      67  2      72  4      79  8      85 16      88 100      94 335     100

2·5 cms. apart

Volts  Current ·5    7·3  1      14  2      27  4      47  8      64 16      73 37·5    81 112      90 375      97 800     100

The results are shown graphically in Fig. 4.

From the above tables it is seen that the current at first increases nearly in proportion to the voltage. There is no evidence of complete saturation, although the current increases very slowly for large increases of voltage. For example, in Table I. a change of voltage from ·125 to ·25 volts increases the current from 18 to 36% of the maximum, while a change of voltage from 100 to 335 volts increases the current only 6%. The variation of the current per volt (assumed uniform between the range of voltages considered) is thus about 5000 times greater for the former change.