McCretitanp— On the Emanation gwen off by Radium. 91 
deflection of the electrometer spot of light. The glass wool in the vessel C stops 
dust particles which might get electrified by friction, and produce a deflection. 
The glass wool also stops the ions which have been produced by the radiation 
from the radium emanation. As the ionised gas has been for some time in B and 
the tube leading to C, there would be a tendency for more negative than positive 
ions to be lost by diffusion to the walls; and the excess of positive would produce 
a deflection when admitted into the vessel D. 
To test whether or not the air thus admitted into D has carried emanation with 
it, and how much, the ionisation current between Zand Dis measured immediately 
after the gas is admitted to D. To do this ¢ is disconnected from ), and joined to 
d. The terminal 7 is thus kept at a high potential; and the air in D being kept 
ionised by radiation from the emanation, the vessel D will gradually be charged ; 
and the rate of charging is measured by the rate of movement of the spot of light 
when the connexion between a and 6 is broken. 
Before the emanation is admitted there is only a very small current to D, 
when 7’ is connected to the storage-battery, this small current being due to the 
weak ionisation which is always present in atmospheric air. 
We thus, by one experiment, measure the charge (if any) carried by the 
emanation; and by a second experiment we measure the ionising power of this 
emanation. 
The Observations. 
We shall now give the numbers obtained in an experiment similar to a great 
number of others carried out. 
The capacity of the electrometer and the necessary connexions, including the 
vessel D, was 131 electrostatic units, or ‘000145 microfarad, and the electrometer 
gave a deflection of 60 scale-divisions for 1 volt difference of potential between its 
quadrants. . 
The admission of the radium emanation produced a deflection of only 4 scale- 
divisions. 
The ionisation current to D was then measured as described above, immediately 
after the admission of the emanation. To sufficiently reduce the rate of movement 
of the spot of light a capacity of ‘1 microfarad was joined to the electrometer, 
and the deflection was then 100 scale-divisions in 47 seconds. 
The admission of the air containing emanation into the vessel D produced, as 
stated, a small deflection of 4 scale-divisions. Preliminary observations had been 
made to see if any deflection would be produced when an equal quantity of air, 
free from emanation, was admitted in the same way. It was found that a small 
deflection was produced probably by some friction effect, the deflection varying in 
different experiments between 0 and 5 scale-divisions, and being always in the 
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