34 ROYAL SOCIETY OF CANADA 



In differentiating the effects due to the various types of rays, 

 the action of each was ascertained by deflecting pencils of each type 

 into an ionisation chamber away from the others by means of a 

 magnetic field. 



II. Apparatus: — The arrangement adopted is shewn in Fig. I. 

 The receptacle for holding the radium was a lead cylinder A with walls 

 4 cm. thick. One end of the cylinder was covered by a brass plate 2.5 

 mms. thick, in the centre of which was a hole 4.5. mms. in diameter. 

 The capsule holding the radium was held close against this plate in 

 such a position that the rays from the radium on issuing, passed 

 between the poles of a powerful electromagnet. Immediately beneath 

 the pole pieces of the electromagnet was the ionisation chamber B. It 

 also was made of lead in the form of a cylinder 6.7 cms. long, with 

 walls 4.6 cms. in thickness, the ionisation chamber proper being 4.7 

 cms. in diameter. At the top and bottom were brass rings to hold 

 different thicknesses of selected absorbing materials over the ends of 

 the chamber. 



A properly screened and insulated electrode was inserted into the 

 ionising chamber and connected to a Dolezeleck electrometer by means 

 of which saturation currents were measured. The sensibility of the 

 electrometer was such that a potential difference of one volt between 

 the quadrants produced a deflection of 625 mms. on a scale about one 

 metre distant from the needle. It was found that a potential of 240 

 volts applied to the ionisation chamber was always amply sufficient 

 to give the saturation current. 



Til. Experiments on the Absorption and Reflection of ft rays by 

 tinfoil. 



A. Measurements on transmitted rays. 



In these experiments the /> rays from the radium, on coming 

 between the poles of the magnet, were deflected either downwards or 

 upwards according to the direction of the field between the poles. 

 As the capsule containing the radium was covered by a thin sheet 

 of mica, the oc rays were largely absorbed, so that the issuing beam 

 contained only ft and y rays, which could easily be separated by 

 the magnetic field in the manner indicated. Readings were taken of 

 the saturation currents in the ionisation chamber as the current 

 through the electromagnet was changed by small increments from to 

 about 28 amperes. 



A series of measurements was made with a number of different 

 thicknesses of absorbing layers of tin foil over the top of the ionisation 

 chamber, and with the bottom of the chamber closed by a thin sheet 

 of aluminium foil, .0065 mms. in thickness. 

 1 Before making these, however, a set of readings was taken with- 



