Emanation given off by Radium. oad 
is removed by a string from a distance, so as not to disturb 
the electrometer by induction effects. The mercury cups 0 
and ¢ in the same block of paraffin are joined to D and T; 
while a fourth cup d is joined to one pole of a battery of small 
storage-cells, the other pole of which is to earth. The vessel 
D is screened from outside electrical disturbances by a 
surrounding earthed conductor not shown in the diagram. 
Method of Working. 
The radium emanation passes readily through the slip of 
thin paper covering the vessel R; and thus, after A has 
remained closed for a short time, it contains a large quantity 
of emanation. The tap ¢, being closed, the vessels B, C, and 
D are partially exhausted to any desired pressure ; ¢; is then 
closed, and ¢, and f, opened ; and thus the vessel B is filled 
with air containing radium emanation, 4 and ¢ being then 
closed again. The mercury cups J and ¢ being joined 
together, the piece connecting a to b is removed, and the 
emanation allowed to rush into D by opening the tap ¢. If 
this emanation carries a charge, it will be shown by a 
defiexion 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 deflexion. The glass-wool also 
stops the ions which have been produced by the radiation 
from the radium emanation. As the ionized 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 pro- 
duce a deflexion 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 ionization 
current between T and D is measured immediately after the 
gas is admitted to D. To do this ¢ is disconnected from b 
and joined to d. The terminal T is thus kept at a high 
potential, and the air in D being kept ionized by radiation 
from the emanation, the vessel D will gradually be charged ; 
and the rate of charging is measured by the rate of move- 
ment of the spot of light when the connexion between a and 
is broken. 
Before the emanation is admitted there is only a very small 
current to D, when T is connected to the storage- -battery, 
this small current being due to the weak ionization 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 ionizing power of this emanation. 
