90 McCrettanp—On the Emanation given off by Radium. 
exhausted, the exhaustion being produced by applying a pump at G. The letters 
t,, 4, and ¢, denote taps by which the tubes can be closed at the points indicated. 
E is a quadrant electrometer, one pair of quadrants being permanently to earth, 
and the other pair joined to a mercury cup 0 in a block of paraffin. The cup d 
is kept connected to an earthed cup a, except when an observation is to be taken, 
ie 
Tae EARTH : 
c d a ie 
Toe amy SU 
EARTH 
and then the connecting piece is removed by a string from a distance, so as not 
to disturb the electrometer by induction effects. 'The mercury cups 2 and ¢ in the 
same block of paraffin are joined to D and 7’; 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; 7; is then closed, and 7, and 4 
opened ; and thus the vessel B is filled with air containing radium emanation, 4 
and % being then closed again. ‘The mercury cups 6 and ¢ being joined together, 
the piece connecting @ to 6 is removed, and the emanation allowed to rush into D 
by opening the tap 4. If this emanation carries a charge, it will be shown by a 
