OF C4ASES UNDER THE BECQUEREL RAYS. 
527 
but the inside is turned down until it is very thin—less than h milllin. Into the 
circidar recess thus formed is dropped a flat brass plate, e, of such a size as to fit it 
loosely. The whole arrangement is fixed horizontally between the pillars carrying 
the quadrants. For fixing it a screw, h, is provided, as a prolongation of the pillar, a. 
This screw passes through a hole drilled centrally in the base of the instrument, and 
a nut fixes the whole in position. 
On e is placed a platinum crucible containing strong suljihuric acid, into which the 
]n'olonged axis of the needle dips as usual. A charge is given to the top brass plate, 
which is, of course, in conducting communication witli the needle. The instrument 
can then be used as usual. 
The insulation of this condenser has been found very satisfactory, the charge not 
diminislhng by more than 20 per cent, of its original value in a week, and this when 
the charge was sufficient to make the sensitiveness very high. 
One or two remarks may be made in conclusion. If the insulation shows a 
tendency to deteriorate with time, it can be made as good as ever by removing the 
surface of the thick liin of ebonite in the lathe, with emery paper, subsequently 
polishing the surface with bath-brick, applied on felt. 
A platinum cup was employed for the acid, with the idea that with it there would 
be no tendency to any creeping of electrification, and consequent uncertainty in the 
potential of the needle. But very probably glass would be practically as good. 
A platinum wire dipping in the acid would then have to be connected to the u})per 
brass plate. 
Although there is no novelty in principle in this contrivance, yet it has proved so 
convenient in practice that it has been thought worth while to describe it. 
