Ionization produced in Gases by Réntgen Rays. 91 
A slightly different form of standard apparatus for testing 
the intensity of the rays was used in this case also. A 
diagram of it is shown in fig. 5. The electrodes in this 
Fig. 5. 

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case consisted of extremely thin aluminium foil stretched on 
copper frames, and the rays passed through at right-angles 
to the plane of the foil. This form was adopted in order 
that the standard apparatus might be made to occupy less 
space, so that the cylinder might be placed as close up to the 
source of rays as possible. This standard apparatus was 
arranged as shown in fig. 4. One electrode was connected 
to the same pole of the battery as the electrode E’. The 
other electrode was connected in parallel with a condenser © 
through a key W to the electrometer. This key was worked 
at a distance by means of a cord, and could be opened at the 
same instant as the key by which E was insulated. 
