[pound] absorption OF THE DIFFERENT TYPES OF BETA RAYS 65 



ionisation chamber when this double thickness of lead and aluminium 

 was placed over the top. The conductivities in the chamber for 

 gradually increasing fields in both directions were taken and these 

 are given in Table XVI. The second column of this Table shews 

 a slight gradual decrease in ionisation as the fi rays were deflected 

 into the chamber, and the fourth column shews a greater decrease as 

 the fi rays were deflected away from the chamber. Here again it 

 will be seen that the difference in the ionisations for the directions 

 of the magnetic field was approximately of the same magnitude 

 when there was 8.4 mms. of aluminium over the ionisation chamber. 

 Since none but x rays could enter the chamber this difference in 

 ionisation must have been due to the action of the magnetic field in 

 the chamber on the secondary radiation issuing from the back of the 

 aluminium screen under the excitation of the /S rays. 



2. The next experiment was to place the radium protected by 

 the lead cylinder on the side of the ionisation chamber directly opposite 

 to its former position. The same aluminium screen 8.14 mms. in 

 thickness was placed over the chamber as before, and the /3 rays were 

 again deflected down into the chamber by a suitably directed magnetic 

 field and afterwards upward and away from it with the field reversed. 

 The results are given in Table XVII. The numbers thus recorded shew 

 the same characteristics as when the radium was in the first position. 

 When the magnetic fields were such as to deflect the /S rays downwards 

 into the chamber the ionisation decreased but slightly. On the other 

 hand a considerably greater decrease took place when the /3 rays 

 were deflected in the opposite direction. 



3. A third experiment was carried out with the radium and its 

 lead protection placed back in the original position. One sheet of 

 tinfoil .0196 mms. in thickness was inserted over the top of the ionisa- 

 tion chamber and 8.14 mms. of aluminium was then placed over the 

 tin. Readings were then taken of the conductivity in the chamber 

 for the two deflections. These readings are given in Table XVIII, and 

 the curves A and A' representing them are drawn in Fig. 16. These 

 curves and the curves E and E' drawn in Fig. 13 are on the same 

 scale. A comparison of the latter which correspond to a screen of 

 8.14 mms. of aluminium alone over the top of the chamber, with the 

 curves A and A' in Fig. 16, makes it clear that the insertion of the sheet 

 of tinfoil beneath the aluminium screen brought the curves repre- 

 senting the two deflections more nearly into coincidence. The 

 natural conclusion would be then, that for a greater thickness of tin- 

 foil below the aluminium the two curves representing the ionisations 

 for the two deflections would coincide. To test this connection four 



Sec. III., 1908. 5. 



