200 RADIO-ACTIVE EMANATIONS [CH. 
an electrometer. An insulating key was arranged so that any one 
of the electrodes H, F, H could be rapidly connected with one pair 
of quadrants of the electrometer, the other two being always con- 
nected with earth. The current observed in the testing cylinder 
vessel was due entirely to the ions produced by the emanation 
carried into the vessel by the current of air. On substituting a 
uranium compound for the thorium, not the slightest current was 
observed. After a constant flow has passed for about 10 minutes, 
the current due to the emanation reaches a constant value. 
The variation of the ionization current with the voltage is 
similar to that observed for the gas ionized by the radiations from 
the active bodies. The current at first increases with the voltage, 
but finally reaches a saturation value. 
132. Duration of the activity of the emanation. The 
emanation rapidly loses its activity with time. This is very readily 
shown with the apparatus of Fig. 37. The current is found to 
diminish progressively along the cylinder, and the variation from 
electrode to electrode depends on the velocity of the flow of air. 
If the velocity of the air current is known, the decay of activity 
of the emanation with time can be deduced. If the flow of air is 
stopped, and the openings of the cylinder closed, the current 
steadily diminishes with time. The following numbers illustrate 
the variation with time of the saturation current, due to the 
emanation in a closed vessel. The observations were taken suc- 
cessively, and as rapidly as possible after the current of air was 
stopped. 
Time in seconds Current 
0 100 
28 69 
62 51 
118 25 
155 14 
210 6°7 
272 4°] 
360 1°8 
Curve A, Fig. 38, shows the relation existing between the 
current through the gas and the time. The current just before 
the flow of air was stopped is taken as unity. The current through 
