66 ROYAL SOCIETY OF CANADA 



sheets of tinfoil or a thickness of .0784 mms. were placed above the 

 opening of the chamber and over this the 8.14 mms. of aluminium. 

 The conductivity in the chamber was then measured for different 

 magnetic fields. Tiie results are given in Table XIX, and curve B 

 illiiltrating them is shewn in Fig. 16. The numbers in the table and 

 the curve both shew that with a screen made up in this way the con- 

 ductivities in the chamber were identical for magnetic fields of equal 

 intensity in either direction. This experiment shewed clearly that 

 the effect under consideration was due to a peculiarity in the secondary 

 radiation emitted by the aluminium. 



4. In the fourth experiment the radium protected by the lead 

 cylinder was placed vertically above the ionisation chamber and also 

 above the poles of the magnet in such a way that the pencils of rays 

 from the radium were directed straight into the ionising chamber. 

 The saturation currents for magnetic fields in both directions when the 

 8.14 mms. of aluminium alone covered the chamber were then 

 measured. From the values of these currents which are recorded in 

 Table XX it will be seen that the ionisation corresponding to any 

 selected field intensity was the same for both directions of the field. 

 Since the disposition of the apparatus in this experiment was symme- 

 trical it follows that the effect noted with the previous arrangement 

 was not only connected with some special property of the secondary 

 radiation excited in the aluminium by the gamma rays, but it also 

 was due evidently to a non-symmetrical configuration of these 

 secondary rays in the ionising chamber. 



The following is offered as an explanation of the foregoing ex- 

 periments. 



In Fig. 17, A represents the ionisation chamber, B the electrode, 

 CD the aluminium screen, and R the position of the radium in the 

 non-symmetrical arrangement. From a consideration of the figure 

 it is evident that the line RA, which is the axis of a pencil of y rays 

 entering the chamber will mark the line of greatest intensity of these 

 rays, since for all other rays the metal path traversed is longer and 

 consequently the absorption is greater. It follows then that RA will 

 also represent the direction of the axis of the pencil of secondary rays 

 of greatest intensity issuing from the back of the aluminium plate. 

 If then the magnetic field was applied in such a direction as to deflect 

 the primary /3 rays down into the chamber, this field since the chamber 

 was so situated as to be affected by the field, would deflect the 

 secondary rays issuing from the back of the aluminium screen in the 

 same direction. With the field in the opposite sense the primary y8 

 rays would be deflected upwards and away from the chamber, and 

 the secondary rays in the chamber would also be turned by this field 



