﻿STRUCTURE 
  OF 
  MATTER 
  — 
  COMPTON. 
  147 
  

  

  The 
  anode 
  rays 
  are 
  also 
  deflected 
  by 
  magnetic 
  and 
  electric 
  fields 
  in 
  

   a 
  direction 
  showing 
  that 
  they 
  are 
  positively 
  charged 
  particles 
  and 
  by 
  

   an 
  amount 
  showing 
  that 
  the 
  ratio 
  of 
  their 
  charge 
  to 
  mass 
  is 
  char- 
  

   acteristic 
  of 
  atoms 
  or 
  molecules 
  of 
  the 
  gas 
  in 
  the 
  tube. 
  In 
  other 
  

   words, 
  they 
  are 
  the 
  residues 
  of 
  the 
  gas 
  atoms 
  or 
  molecules 
  which 
  

   remain 
  after 
  electrons 
  have 
  been 
  driven 
  out. 
  Knowing 
  their 
  charges, 
  

   the 
  bending 
  of 
  their 
  paths 
  in 
  magnetic 
  and 
  electric 
  fields 
  enables 
  

   their 
  masses 
  to 
  be 
  determined. 
  It 
  is 
  in 
  this 
  manner 
  that 
  atoms 
  and 
  

   molecules 
  have 
  been 
  individually 
  weighed 
  with 
  high 
  precision. 
  

  

  In 
  order 
  to 
  find 
  the 
  mass 
  m 
  from 
  the 
  above 
  values 
  of 
  e/m, 
  it 
  is 
  

   necessary 
  to 
  know 
  the 
  charge 
  e 
  of 
  a 
  negative 
  electron. 
  This 
  has 
  been 
  

   measured 
  with 
  the 
  greatest 
  accuracy 
  by 
  Professor 
  Millikan 
  about 
  

   eight 
  years 
  ago. 
  The 
  most 
  sensitive 
  instrument 
  for 
  measurement 
  of 
  

   electric 
  charges 
  is 
  the 
  electroscope, 
  which 
  consists, 
  essentially, 
  of 
  a 
  

   strip 
  of 
  gold 
  leaf 
  suspended 
  between 
  two 
  oppositely 
  charged 
  metal 
  

   plates. 
  When 
  the 
  gold 
  leaf 
  is 
  charged 
  it 
  is 
  attracted 
  by 
  one 
  plate 
  

   and 
  repelled 
  by 
  the 
  other, 
  and 
  the 
  size 
  of 
  its 
  charge 
  may 
  be 
  measured 
  

   by 
  observing 
  the 
  distance 
  which 
  it 
  moves 
  from 
  its 
  uncharged 
  position. 
  

   But 
  this 
  instrument 
  is 
  not 
  sensitive 
  enough 
  to 
  measure 
  the 
  charge 
  

   of 
  an 
  electron. 
  Professor 
  Millikan 
  substituted 
  for 
  the 
  gold 
  leaf 
  a 
  

   tiny 
  droplet 
  of 
  oil 
  from 
  the 
  spray 
  of 
  an 
  atomizer. 
  Because 
  of 
  its 
  

   weight 
  it 
  tended 
  to 
  fall 
  through 
  the 
  air 
  slowly, 
  because 
  of 
  its 
  small 
  

   size 
  and 
  the 
  viscous 
  resistance 
  offered 
  by 
  the 
  air 
  ; 
  but 
  if 
  this 
  droplet 
  

   were 
  electrically 
  charged, 
  it 
  could 
  be 
  drawn 
  upward, 
  in 
  opposition 
  to 
  

   gravitjr, 
  bjr 
  an 
  electric 
  field 
  between 
  the 
  two 
  horizontal 
  metal 
  plates 
  

   between 
  which 
  the 
  droplet 
  moved. 
  By 
  observing 
  through 
  a 
  telescope 
  

   the 
  rate 
  at 
  which 
  the 
  drop 
  fell 
  in 
  the 
  absence 
  of 
  an 
  electric 
  field 
  and 
  

   the 
  rate 
  at 
  which 
  it 
  rose 
  in 
  the 
  field, 
  data 
  were 
  obtained 
  permitting 
  

   a 
  calculation 
  of 
  the 
  amount 
  of 
  electric 
  charge 
  on 
  the 
  drop. 
  It 
  was 
  

   found 
  that 
  all 
  charges 
  were 
  simple 
  multiples 
  of 
  a 
  fundamental 
  unit 
  

   charge, 
  which 
  is 
  the 
  charge 
  of 
  an 
  electron. 
  Thus 
  the 
  negative 
  elec- 
  

   tron 
  is 
  not 
  only 
  a 
  fundamental 
  unit 
  of 
  matter 
  but 
  also 
  a 
  fundamental 
  

   unit 
  of 
  electricity. 
  

  

  By 
  such 
  experiments 
  it 
  is 
  found 
  that 
  the 
  mass 
  of 
  a 
  negative 
  elec- 
  

   tron 
  is 
  8.97 
  ( 
  10) 
  -28 
  grams 
  and 
  its 
  charge 
  is 
  4.774 
  ( 
  10) 
  _1 
  ° 
  electrostatic 
  

   units. 
  The 
  mass 
  of 
  a 
  hydrogen 
  atom 
  is 
  1.65 
  ( 
  10) 
  " 
  24 
  grams. 
  

  

  Positive 
  electrons. 
  — 
  When 
  the 
  positively 
  charged 
  residue 
  of 
  an 
  

   atom, 
  the 
  part 
  left 
  after 
  the 
  loss 
  of 
  an 
  electron, 
  is 
  weighed 
  by 
  meas- 
  

   uring 
  the 
  bending 
  of 
  its 
  path 
  in 
  an 
  electric 
  and 
  magnetic 
  field, 
  two 
  

   very 
  significant 
  results 
  are 
  obtained. 
  In 
  the 
  first 
  place, 
  the 
  weight 
  

   of 
  every 
  atom, 
  except 
  hydrogen, 
  is 
  an 
  exact 
  integral 
  multiple 
  of 
  the 
  

   weight 
  of 
  a 
  fundamental 
  unit. 
  This 
  unit 
  is 
  one-fourth 
  the 
  weight 
  

   of 
  a 
  helium 
  atom, 
  or 
  one-twelfth 
  that 
  of 
  a 
  carbon 
  atom, 
  or 
  one- 
  

   sixteenth 
  that 
  of 
  an 
  oxygen 
  atom, 
  etc. 
  The 
  unit 
  has 
  almost 
  the 
  

   weight 
  of 
  a 
  hydrogen 
  atom, 
  but 
  is 
  less 
  by 
  0.77%. 
  This 
  discrep- 
  

  

  