572 Prof. E. Kutherford and Mr. F. Soddy on 



length and 6 cm. in diameter. The cylinder insulated on 

 paraffin blocks was connected to one pole of a battery of 

 small lead accumulators, the other pole of which was con- 

 nected to earth. Three electrodes E, F, H, of equal length 

 were placed along the axis of the cylinder. The current 

 through the gas was measured by means of a Kelvin electro- 

 meter of the White pattern. The electrometer and con- 

 nexions were suitably screened by means of wire gauze con- 

 nected to earth. An insulating key was arranged so that 

 either of the electrodes E, F, H, or all of them together, 

 could be rapidly connected to one pair of quadrants of the 

 electrometer, the other two being always connected to earth. 

 The measurements were carried out in the usual way by 

 observing the rate of movement of the electrometer-needle 

 after the one pair of quadrants were connected with the 

 electrodes. On placing the emanating substance in and 

 continuing the air current for several minutes at a constant 

 rate, the current through the gas due to the emanation 

 attains a steady state. The number of divisions of the scale 

 passed over per second may be taken as a measure of the 

 current. 



With this apparatus the emanation from 10 grams of 

 ordinary thorium oxide produces a current of 3*3 x 10 -11 

 amperes between the three electrodes connected together and 

 the cylinder. With the electrometer working at an average 

 sensitiveness, this corresponded to a deflexion of 100 divisions 

 of the scale in 12 seconds, so that one-hundredth part of this 

 current could be readily measured — that is, the emanation 

 produced by one-tenth of a gram of thorium oxide. The 

 electrometer one hundred times more sensitive than this 

 failed to detect the presence of an emanation or radioactivity 

 in the oxides of tin, zirconium, and titanium, the other elements 

 of the same group in the periodic table. 



Rate of Decay of the Radiation from the Emanation. — The 

 three electrodes E, F, H, were used to compare the " rates of 

 decay " of the radiations from the emanations of different 

 substances. In the previous papers quoted, it has been shown 

 that the radiating power of the thoria emanation falls to half 

 its value in about a minute. In consequence of this, the 

 current observed for the electrode E is greater than for the 

 electrode H. Knowing the velocity of the current of air 

 along the cylinder and the respective currents to the elec- 

 trodes E, F, H, the rate of decay of the radiation can be 

 readily deduced. If, however, we merely require to compare 

 the rate of decay of one emanation with another, it is only 

 necessary to compare the ratio of the currents to the elec- 



