﻿720 
  Prof. 
  E. 
  Rutherford 
  and 
  Mr. 
  H. 
  Robinson 
  on 
  the 
  

  

  Drass 
  vessel 
  was 
  placed 
  between 
  the 
  parallel 
  pole-pieces 
  

   (17 
  X 
  10 
  cm.) 
  of 
  a 
  large 
  electromagnet. 
  The 
  pole-pieces 
  were 
  

   adjusted 
  so 
  that 
  the 
  magnetic 
  field 
  was 
  very 
  nearly 
  uniform 
  

   over 
  the 
  greater 
  part 
  of 
  the 
  space 
  between 
  them. 
  The 
  

  

  Fig-. 
  1. 
  

  

  magnetic 
  field 
  was 
  parallel 
  to 
  the 
  source 
  and 
  slit. 
  In 
  the 
  

   uniform 
  magnetic 
  field 
  the 
  /5 
  particles 
  of 
  a 
  definite 
  velocity 
  

   describe 
  circles 
  of 
  the 
  same 
  radius. 
  It 
  can 
  be 
  shown 
  theo- 
  

   retically 
  that 
  even 
  with 
  a 
  comparative^ 
  wide 
  slit 
  the 
  circles 
  

   intersect 
  the 
  photographic 
  plate 
  along 
  a 
  curved 
  line 
  of 
  

   comparatively 
  narrow 
  width. 
  This 
  result, 
  however, 
  is 
  most 
  

   clearly 
  shown 
  by 
  drawing 
  circular 
  paths 
  of 
  /3 
  rays 
  of 
  definite 
  

   velocity. 
  It 
  is 
  seen 
  from 
  fig. 
  1 
  that 
  if 
  the 
  pencil 
  of 
  rays 
  

   comprised 
  (3 
  rays 
  of 
  only 
  one 
  velocity, 
  the 
  whole 
  pencil 
  of 
  

   rays 
  would 
  be 
  concentrated 
  along 
  a 
  narrow 
  line 
  on 
  the 
  

   photographic 
  plate, 
  and 
  consequently 
  the 
  photographic 
  im- 
  

   pression 
  for 
  a 
  given 
  exposure 
  would 
  be 
  far 
  more 
  confined 
  

   and 
  intense 
  than 
  if 
  the 
  photographic 
  plate 
  were 
  placed 
  

   vertically 
  above 
  the 
  slit 
  where 
  the 
  cone 
  of 
  rays 
  is 
  com- 
  

   paratively 
  wide. 
  

  

  It 
  is 
  clear 
  from 
  the 
  figure 
  that 
  this 
  concentration 
  holds 
  

   for 
  /3 
  rays 
  of 
  different 
  speeds. 
  Special 
  precautions 
  were 
  

   taken 
  to 
  reduce 
  the 
  effect 
  of 
  the 
  scattered 
  /3 
  radiations 
  which 
  

   must 
  necessarily 
  be 
  present. 
  Apart 
  from 
  the 
  question 
  

  

  