5-kO Mr. G. H. Henderson on the Range and Ionization 



part in eleven hundred. The thickness of the diaphragm 

 was sufficient to absorb ft rays. Those passing through the 

 opening were bent away from the ionization chamber by a 

 magnetic field, which also served to curl up the paths of any 

 8 particles produced in the chamber. The pole-pieces of the 

 electromagnet are indicated by the dotted rectangle in fig. 1 . 

 Two other diaphragms were also inserted to prevent a. 

 particles from entering the ionization chamber after meeting 

 the walls of the glass vessel A and there undergoing large 

 angle scattering. 



Fine adjustment of the air pressure in A was secured 

 by the movable mercury cistern H and the arrangement 

 shown in fig. 1. The pressure was measured hy a mano- 

 meter of 1*7 cm. diameter, this large diameter being used to 

 reduce capillary effects. The heights of the mercury 

 columns were observed on a cathetometer on which readings 

 could be estimated to '025 mm. Thus the error in the 

 pressure measurement was not more than *05 mm. The 

 temperature of the air within the vessel was given by the 

 thermometer F, graduated to *2° C. To minimize tempera- 

 ture variations the vessel A was wound closely along its 

 length by lead tubing containg circulating water. 



The distance from the source to the centre of the ionization 

 chamber was 38'12 cm., so that the depth of the chamber 

 was equivalent to about g-l- of the range of the a particles, 

 i. e., about '02 cm. for radium in air at n.t p. Fine 

 " resolving power " of the apparatus was thus secured by the 

 shallowness of the ionization chamber. 



The radioactive sources employed were usually equivalent 

 in 7 ray activity to about 5 mg. of radium. On account of 

 the shallowness of the ionization chamber, and also on 

 account of the small permissible solid angle of the bundle of 

 a. rays, the ionization current was small. To measure this 

 current a Compton * quadrant electrometer was constructed 

 with tilted needle and movable quadrant. The writer 

 wishes to take this opportunity to thank Dr. A. H. Compton 

 for many valuable suggestions during the setting up of this 

 instrument. Though capable of much higher sensitivities 

 this electrometer was usually used at a sensitivity of about 

 5000 divisions per volt, when it was very stable and readings 

 could be taken quickly. 



The procedure during a series of observations with radium 

 C was as follows. The polished brass plate was exposed 

 to radium emanation for about two and a half hours and 



* A. H. and K. T. Compton, Phys. Kev. xiv. p. 85 (1919). 



