820 Mr. E. W. Varder and Dr. E. Marsden on the 



A pencil of a particles from the source S emerged from 

 the brass tube T and fell on the usual Bragg ionization 

 apparatus B. The current between the high potential gauze 

 (x and the electrode E was measured by a Wilson-Kaye 

 electroscope. The distance from S to E could be varied by 

 the ratchet R and registered on the scale M. Jn this way 

 an ionization curve could be obtained. Under ordinary 

 circumstances the ionization at distances greater than 5'4cm. 

 is about 2 per cent, of the maximum ionization of the curve. 

 As the effect to be expected was less than 1/10 of this, it 

 was necessary to use a magnetic field sufficiently strong to 

 turn aside all 13 rays. To obtain such a field over the largo 

 area and gap necessary to accommodate the apparatus, a 

 large dynamo was dismantled and its field windings suitably 

 adapted. It was found that a field of 2500 gauss was 

 sufficient, larger fields causing no reduction in the ionization. 

 In this way the curves shown in fig. 2 were determined. 



Fig. 2. 



0-60 



0-40 



$CT/NtUMC 



qc 0'20 



5-4 53 5-8 7-0 7-Z So 



Distance from source, Cms. (15° C. and 76 cm. Hg). 



In the case of actinium only a few of the end points could 

 be obtained with any certainty, owing to the small intensity 

 of the best preparation obtainable and the rapid decay of the 

 source during an experiment. The electroscope was used 

 at as high a sensibility as possible, over 150 scale-divisions 

 per volt. 



However, the essential difference between the three curves 

 is sufficiently apparent, the curve for actinium indicating 

 clearly the presence of longer range a particles than those 

 of range 5*4 cm. To obtain an estimate of the ionization 



