584 



BELL SYSTEM TECHNICAL JOURNAL 



2 l-l 



;0.8 



'0.7 

 0.6 

 0.5 

 0.4 

 0.3 

 0.2 

 0.1 



9.2 9.4 9.6 



10.0 10.2 10.4 10.6 10.8 1 1.0 11.2 

 RANGE IN AIR IN CENTIMETERS 



11.4 11.6 11.8 12.0 12.2 



Fig. 5 — Distribution-in-range of the long-range alpha-particles proceeding from 

 RaC', determined with the differential ionization-chamber (Rutherford Ward & 

 Lewis; Froc. Roy. Soc). 



that their disc-shaped faces were reduced to narrow circular rings, 

 there would be a great economy in magnetizable metal and a great 

 reduction of weight and volume of the apparatus, as well as other 

 advantages. This need not impair the availability of the magnet for 

 analyzing a beam of a-particles, provided that the a-emitter can be 

 located in the narrow annular space between the faces of the rings, and 

 provided that the magnetization of the metal can be varied sufficiently 

 widely. For then, for each group of a-particles there will be a certain 

 value of the field-strength, whereby those particles which start out in 

 directions nearly perpendicular to the field and tangent to the rings 

 will be swept around in circular paths which are confined within that 

 narrow an,nular space where alone the field exists. Somewhere in that 

 space the detector should be placed; and the curve of its reading vs. 

 field-strength H should show a peak for every group, and from the 

 abscissa of the peak and the radius of the rings the energy of the group 

 may readily be computed. 



Such a magnet was built after Cockcrof t's design at the Cavendish ; 

 the radius of its rings is 40 cm., they are 5 cm. broad and 1 cm, apart 

 (these figures are the dimensions of the annular space), and the field- 

 strength was adjustable up to 10,000 (later 12,000) gauss which was 



