﻿722 
  Prof. 
  E. 
  Rutherford 
  and 
  Mr. 
  H. 
  Robinson 
  on 
  the 
  

  

  about 
  200 
  gauss 
  to 
  6300 
  gauss. 
  In 
  all, 
  more 
  than 
  fifty- 
  

   separate 
  photographs 
  were 
  taken. 
  

  

  The 
  apparatus 
  described 
  above 
  could 
  be 
  conveniently 
  used 
  

   up 
  to 
  fields 
  of 
  3500 
  gauss. 
  For 
  the 
  higher 
  fields 
  a 
  similar 
  

   apparatus 
  to 
  that 
  shown 
  in 
  fig. 
  1 
  was 
  used, 
  but 
  with 
  an 
  

   internal 
  breadth 
  of 
  only 
  1*9 
  cm. 
  instead 
  of 
  3'2 
  cm. 
  Another 
  

   similar 
  electromagnet 
  was 
  used 
  and, 
  with 
  the 
  narrower 
  gap, 
  

   constant 
  fields 
  of 
  over 
  7000 
  gauss 
  could 
  be 
  obtained 
  without 
  

   excessive 
  heating 
  of 
  the 
  magnet. 
  

  

  Each 
  group 
  given 
  in 
  the 
  Tables 
  I. 
  and 
  II. 
  was 
  observed 
  in 
  

   widely 
  differing 
  positions 
  on 
  three 
  different 
  plates. 
  It 
  was 
  

   possible 
  from 
  this 
  circumstance 
  to 
  check 
  the 
  accuracy 
  of 
  the 
  

   measurements 
  of 
  the 
  magnetic 
  fields, 
  and 
  also 
  the 
  uniformity 
  

   of 
  the 
  magnetic 
  field 
  over 
  the 
  whole 
  region 
  between 
  the 
  

   poles. 
  It 
  was 
  usually 
  found 
  that 
  when 
  a 
  line 
  appeared 
  on 
  

   the 
  plate 
  at 
  a 
  distance 
  of 
  more 
  than 
  7 
  cm. 
  from 
  the 
  slit, 
  the 
  

   measurement 
  gave 
  slightly 
  too 
  high 
  values 
  of 
  the 
  radius 
  of 
  

   curvature, 
  on 
  account 
  of 
  the 
  /3 
  particle 
  entering 
  the 
  outer 
  

   and 
  non-uniform 
  regions 
  of 
  the 
  magnetic 
  field. 
  All 
  the 
  final 
  

   measurements 
  were 
  confined 
  to 
  lines 
  appearing 
  on 
  the 
  photo- 
  

   graphic 
  plate 
  within 
  6 
  cm. 
  from 
  the 
  slit. 
  The 
  uniformity 
  of 
  

   the 
  magnetic 
  field 
  in 
  both 
  vertical 
  and 
  horizontal 
  directions 
  

   was 
  carefully 
  tested 
  by 
  means 
  of 
  a 
  small 
  search 
  coil, 
  and 
  the 
  

   positions 
  of 
  the 
  poles 
  so 
  adjusted 
  as 
  to 
  give 
  equality 
  of 
  field 
  

   over 
  a 
  considerable 
  area. 
  The 
  field 
  itself 
  was 
  measured 
  by 
  

   standard 
  coils 
  and 
  a 
  ballistic 
  galvanometer 
  in 
  the 
  usual 
  way. 
  

   Measurements 
  of 
  the 
  intensity 
  of 
  the 
  magnetic 
  field 
  for 
  a 
  

   given 
  current 
  gave 
  results 
  consistent 
  to 
  1 
  part 
  in 
  300, 
  

   provided 
  the 
  iron 
  was 
  brought 
  into 
  a 
  cyclic 
  state 
  by 
  a 
  number 
  

   of 
  reversals 
  of 
  the 
  exciting 
  current. 
  This 
  precaution 
  was 
  

   more 
  especially 
  necessary 
  in 
  the 
  case 
  of 
  the 
  weaker 
  fields. 
  

  

  Sources 
  of 
  /3 
  Rays 
  employed. 
  

  

  (1) 
  Thin-walled 
  emanation 
  tube. 
  

  

  (2) 
  Wires 
  coated 
  with 
  radium 
  B 
  and 
  C. 
  

  

  (3) 
  Wires 
  coated 
  with 
  radium 
  alone. 
  

  

  As 
  already 
  mentioned, 
  the 
  high 
  velocity 
  groups 
  of 
  rays 
  

   were 
  first 
  photographed 
  by 
  using 
  an 
  a-ray 
  tube. 
  The 
  thick- 
  

   ness 
  of 
  the 
  glass 
  wall 
  of 
  the 
  tube 
  corresponded 
  in 
  stopping- 
  

   power 
  for 
  the 
  a 
  rays 
  to 
  1*2 
  cm. 
  of 
  air. 
  There 
  was 
  a 
  small 
  

   correction 
  for 
  the 
  decrease 
  of 
  velocity 
  of 
  the 
  /3 
  particle 
  in 
  

   traversing 
  the 
  glass. 
  This 
  was 
  made 
  by 
  observing 
  the 
  shift 
  

   of 
  the 
  lines 
  when 
  a 
  thin 
  mica 
  plate 
  of 
  known 
  thickness 
  was 
  

   placed 
  immediately 
  over 
  the 
  source. 
  The 
  corrections 
  were 
  

  

  