128 Mr. Y. E. Pound on the Absorption of the 



into the ionizing-chamber and connected to a Dolezalek 

 electrometer by means of which saturation currents were 

 measured. The sensibility of the electro meter was such that 

 a potential-difference of one volt between the quadrants 

 produced a deflexion of 625 mms. on a scale about one metre 

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

 210 volts applied to the ionization-chamber was always 

 amply sufficient to give the saturation current. 



III. Experiments on the Absorption and Reflexion 

 of /3 Rays by Tinfoil. 



A. Measurements on Transmitted Rays. 



In these experiments the /3 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 a rays were 

 largely absorbed, so that the issuing beam contained only /3 

 and 7 rays, which could easily be separated by the magnetic 

 field in the manner indicated. Readings were taken of the 

 saturation currents in the ionization-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 tinfoil over the 

 top of the ionization-chamber, and with the bottom of the 

 chamber closed by a thin sheet of aluminium-foil 0*0065 mm. 

 in thickness. 



Before making these, however, a set of readings w r as 

 taken without any metallic covering over the opening at the 

 top of the ionizing-chamber, and with only the single sheet of 

 aluminium -foil over the opening at the bottom. In taking 

 these the magnetic field was first applied in such a direction 

 as to deflect the /3 rays down into the chamber, and observa- 

 tions were made on the saturation currents corresponding to 

 various field- strengths. The field was then reversed, and a 

 second set of readings taken as the j3 rays were gradually 

 deflected upwards and away from the chamber. Both set j 

 of readings are given in column I. of Tables I. and II. 

 respectively, and curves representing them are shown in 

 fig. 2 (PI. II.). From these curves it is seen that as the /3 rays 

 were deflected down into the chamber by the magnetic field, the 

 ionization in the chamber rapidly increased to a maximum 

 value, and then decreased as the different pencils of rays were 

 swept past by the increasing magnetic fields. It is seen, 



