﻿Deposit 
  upon 
  the 
  Poles 
  of 
  Metallic 
  Arcs. 
  341 
  

  

  This 
  would 
  account 
  for 
  a 
  smaller 
  electric 
  force 
  being 
  re- 
  

   quired 
  to 
  attract 
  the 
  positive 
  ions 
  to 
  the 
  cathode 
  than 
  is 
  required 
  

   between 
  the 
  negative 
  ions 
  and 
  the 
  anode. 
  If 
  a 
  difference 
  in 
  

   bulk 
  is 
  one 
  cause 
  of 
  the 
  different 
  actions 
  of 
  the 
  positive 
  and 
  

   negative 
  ions, 
  it 
  is 
  reasonable 
  to 
  assume 
  that 
  the 
  absence 
  of 
  

   a 
  silver 
  deposit 
  is 
  due 
  to 
  its 
  being 
  too 
  heavy 
  to 
  be 
  moved 
  by 
  

   the 
  electric 
  force 
  upon 
  it, 
  although 
  it 
  can 
  be 
  formed 
  upon 
  a 
  

   third 
  pole 
  at 
  a 
  higher 
  potential 
  than 
  can 
  be 
  maintained 
  

   between 
  the 
  poles 
  of 
  the 
  arc 
  itself. 
  The 
  silver 
  cluster 
  would, 
  

   for 
  a 
  given 
  number 
  of 
  molecules, 
  be 
  expected 
  to 
  be 
  heavier 
  

   than 
  the 
  copper 
  and 
  the 
  copper 
  heavier 
  than 
  the 
  iron. 
  This 
  

   has 
  been 
  borne 
  out 
  by 
  the 
  series 
  of 
  experiments 
  in 
  which 
  it 
  

   has 
  been 
  determined 
  that 
  the 
  iron, 
  copper, 
  and 
  silver 
  deposits, 
  

   are 
  formed 
  with 
  greater 
  difficulty 
  as 
  the 
  atomic 
  weight 
  

   increases. 
  

  

  Ehrenhaft's* 
  experiments 
  upon 
  the 
  charge 
  carried 
  by 
  the 
  

   ions 
  derived 
  from 
  metallic 
  arcs 
  necessitated 
  the 
  measure- 
  

   ment 
  of 
  their 
  velocities 
  both 
  in 
  the 
  electric 
  field 
  and 
  under 
  

   the 
  influence 
  of 
  gravitation. 
  His 
  determinations 
  led 
  him 
  to 
  

   the 
  conclusion 
  that 
  the 
  velocities 
  of 
  the 
  positive 
  and 
  negative 
  

   ions 
  are 
  the 
  same 
  to 
  within 
  5 
  per 
  cent, 
  if 
  the 
  mean 
  values 
  be 
  

   taken, 
  but 
  too 
  much 
  stress 
  must 
  not 
  be 
  placed 
  on 
  this 
  con- 
  

   clusion 
  because 
  the 
  velocities 
  of: 
  individual 
  particles 
  varied 
  

   by 
  as 
  much 
  as 
  60 
  per 
  cent. 
  We 
  are 
  not 
  concerned 
  so 
  much 
  

   with 
  the 
  mean 
  velocity 
  as 
  with 
  the 
  maximum 
  velocity 
  with 
  

   which 
  thev 
  can 
  be 
  urged 
  in 
  the 
  electric 
  field, 
  and 
  this 
  might 
  

   easily 
  be 
  different 
  for 
  the 
  two 
  types 
  of 
  ions 
  without 
  greatly 
  

   affecting 
  their 
  mean 
  value. 
  Nevertheless 
  the 
  experiments 
  

   do 
  not 
  confirm 
  the 
  hypothesis 
  that 
  the 
  mobilities 
  of 
  the 
  two 
  

   are 
  different, 
  though 
  the 
  objection 
  is 
  not 
  I 
  think 
  very 
  for- 
  

   midable 
  since 
  Ehrenhaft 
  used 
  an 
  electric 
  field 
  of 
  2"5 
  electro- 
  

   static 
  units, 
  whereas 
  those 
  employed 
  in 
  the 
  above 
  experiments 
  

   were 
  considerably 
  smaller, 
  and 
  it 
  has 
  already 
  been 
  stated 
  that 
  

   the 
  differences 
  between 
  the 
  positive 
  and 
  negative 
  pole 
  rapidly 
  

   diminished 
  as 
  greater 
  potential 
  differences 
  were 
  employed. 
  

  

  (2) 
  Another 
  cause 
  of 
  the 
  differences 
  between 
  the 
  action 
  of 
  

   the 
  two 
  poles 
  is 
  the 
  discharge 
  of 
  electrons 
  from 
  the 
  cathode 
  

   (above 
  the 
  region 
  of 
  the 
  arc 
  proper) 
  both 
  under 
  the 
  influences 
  

   of 
  its 
  high 
  temperature 
  (thermal 
  emission) 
  and 
  of 
  the 
  intense 
  

   ultra-violet 
  light 
  incident 
  upon 
  it. 
  

  

  On 
  this 
  view 
  the 
  stream 
  of 
  electrons 
  provides 
  nuclei 
  for 
  

   the 
  vapour 
  which 
  is 
  not 
  yet 
  condensed 
  from 
  the 
  rising 
  and 
  

   rapidly 
  cooling 
  products 
  of 
  the 
  arc, 
  those 
  particles 
  that 
  

  

  * 
  Ehrenhaft, 
  Phi/s. 
  Zeit. 
  x. 
  p. 
  308 
  (1909). 
  

  

  