170 McCiettaAnp— On Secondary Radiation. 
and 3:2cms. internal diameter, connected to small storage-cells, by which it 
could be kept at any required potential: a metal rod is fixed along the axis of 
the tube, being insulated by paraffin and joined to a Dolezalek electrometer, the 
joining wire being suitably protected. ‘The radium is placed at R so as to send a 
pencil of rays through a hole in a thick lead screen, other lead screens being 
placed to protect the tube 7’ from direct radiation. 
A plate of the substance under examination is placed at P, and the secondary 
radiation from it enters the tube 7’, and produces ionisation, which is measured in 
the usual way by the electrometer. The end of the tube 7 is covered with a 
ELECTROMETER<Z 
Fic. 1. 
single sheet of tinfoil. ‘The distance from the radium to the plate P was in most 
of the experiments about 26cms., and from P to the nearest point of 7’ was 
usually about 9 ems. ‘The distances are given in every case where they are of 
importance. . 
In every experiment the ionisation observed in 7’ is corrected for whatever 
small conductivity existed between the tube 7’ and its inner terminal when the 
plate P is not in position, this small conductivity beng due partly to the normal 
ionisation inside 7’, and partly to insufficient screening of direct radiation from 
