﻿NUCLEUS 
  OP 
  THE 
  ATOM 
  CROWTHER 
  215 
  

  

  sented 
  by 
  so 
  simple 
  a 
  law 
  of 
  force 
  is 
  somewhat 
  improbable. 
  That 
  the 
  

   force 
  should 
  change 
  from 
  one 
  of 
  repulsion 
  to 
  one 
  of 
  attraction 
  seems 
  

   to 
  be 
  necessitated 
  by 
  the 
  very 
  existence 
  of 
  a 
  nucleus. 
  It 
  is, 
  moreover, 
  

   corroborated 
  by 
  some 
  very 
  recent 
  experiments 
  by 
  Rutherford. 
  An 
  a 
  

   particle 
  approaching 
  an 
  aluminum 
  nucleus 
  will 
  be 
  repelled, 
  and 
  will, 
  

   therefore, 
  be 
  spending 
  its 
  energy 
  until 
  the 
  critical 
  circle 
  is 
  reached. 
  

   Once 
  this 
  point 
  is 
  passed 
  it 
  will 
  fall 
  spontaneously 
  into 
  the 
  nucleus, 
  

   gathering 
  fresh 
  energy 
  as 
  it 
  goes. 
  We 
  can, 
  indeed, 
  regard 
  this 
  critical 
  

   circle 
  as 
  a 
  high 
  mountain 
  ridge 
  surrounding 
  a 
  small, 
  deep, 
  circular 
  

   valley 
  in 
  which 
  the 
  nucleus 
  lies. 
  In 
  electrical 
  language 
  the 
  circle 
  is 
  

   one 
  of 
  maximum 
  potential. 
  Now, 
  suppose 
  we 
  fire 
  an 
  a 
  particle 
  up 
  

   this 
  rising 
  slope. 
  If 
  its 
  initial 
  energy 
  is 
  insufficient 
  to 
  carry 
  it 
  to 
  the 
  

   top, 
  it 
  will 
  come 
  momentarily 
  to 
  rest 
  and 
  then 
  roll 
  back 
  again 
  down 
  

   the 
  slope. 
  If, 
  however, 
  it 
  has 
  enough 
  or 
  more 
  than 
  enough 
  energy 
  

   to 
  carry 
  it 
  over 
  the 
  ridge, 
  it 
  will 
  fall 
  into 
  the 
  valley 
  beyond 
  ; 
  that 
  is, 
  

   into 
  the 
  nucleus. 
  What 
  happens 
  under 
  these 
  circumstances 
  was 
  

   already 
  known 
  from 
  earlier 
  experiments. 
  The 
  aluminum 
  nucleus 
  is 
  

   disintegrated 
  and 
  one 
  or 
  more 
  protons, 
  or 
  hydrogen 
  nuclei, 
  are 
  ejected 
  

   from 
  it. 
  A 
  similar 
  artificial 
  disintegration 
  can 
  be 
  produced 
  by 
  the 
  

   same 
  means 
  in 
  other 
  elements 
  of 
  low 
  atomic 
  weight. 
  The 
  minimum 
  

   energy 
  which 
  an 
  a 
  particle 
  must 
  have 
  to 
  produce 
  this 
  disintegration 
  

   must 
  obviously 
  be 
  sufficient 
  to 
  carry 
  it 
  over 
  the 
  crest 
  of 
  the 
  potential 
  

   slope. 
  Direct 
  measurements 
  showed 
  that 
  the 
  disintegration 
  of 
  alumi- 
  

   num 
  was 
  not 
  effected 
  unless 
  the 
  a 
  particles 
  used 
  had 
  an 
  initial 
  energy 
  

   corresponding 
  to 
  that 
  which 
  they 
  would 
  acquire 
  in 
  falling 
  freely 
  

   through 
  a 
  potential 
  difference 
  of 
  2,800,000 
  volts. 
  Neglecting 
  any 
  

   trifling 
  losses 
  of 
  energy 
  which 
  the 
  a 
  particle 
  may 
  undergo 
  in 
  its 
  path 
  

   due 
  to 
  other 
  causes, 
  the 
  height 
  of 
  our 
  mountain 
  ridge 
  must 
  therefore 
  

   be 
  about 
  2,800,000 
  volts 
  in 
  electrical 
  units. 
  

  

  The 
  matter 
  can 
  also 
  be 
  tested 
  from 
  the 
  other 
  side. 
  The 
  proton 
  

   expelled 
  must 
  have 
  come 
  from 
  the 
  valley 
  over 
  the 
  ridge, 
  and, 
  even 
  

   if 
  it 
  topped 
  the 
  ridge 
  with 
  no 
  remaining 
  velocity, 
  its 
  final 
  velocity 
  on 
  

   leaving 
  the 
  atom 
  must 
  be 
  at 
  least 
  that 
  which 
  would 
  be 
  produced 
  by 
  

   a 
  free 
  fall 
  down 
  the 
  outer 
  electrical 
  slope, 
  since 
  in 
  this 
  part 
  of 
  its 
  

   journey 
  it 
  is 
  being 
  vigorously 
  repelled 
  by 
  the 
  nucleus 
  it 
  has 
  left. 
  

   The 
  hydrogen 
  particles 
  will 
  thus 
  be 
  expelled 
  with 
  a 
  minimum 
  energy 
  

   corresponding 
  to 
  that 
  gathered 
  during 
  a 
  free 
  fall 
  through 
  the 
  full 
  

   potential 
  of 
  the 
  critical 
  layer. 
  By 
  actual 
  measurement 
  Rutherford 
  

   found 
  that 
  the 
  minimum 
  energy 
  with 
  which 
  the 
  particles 
  were 
  

   expelled 
  corresponded 
  to 
  a 
  fall 
  through 
  about 
  3,000,000 
  volts. 
  The 
  

   very 
  close 
  agreement 
  of 
  the 
  two 
  estimates 
  affords 
  the 
  strongest 
  evi- 
  

   dence 
  for 
  the 
  actual 
  existence 
  of 
  a 
  surface 
  of 
  maximum 
  potential 
  

   surrounding 
  the 
  nucleus, 
  and 
  for 
  the 
  change 
  in 
  direction 
  of 
  the 
  

   force 
  at 
  this 
  point 
  from 
  wliich 
  it 
  was 
  deduced. 
  

  

  