﻿Positive 
  Electricitij. 
  829 
  

  

  discharges, 
  but 
  also 
  Avhen 
  the 
  electric 
  field 
  is 
  much 
  less 
  

   intense 
  than 
  when 
  the 
  discharge 
  is 
  produced 
  by 
  the 
  induction- 
  

   coil. 
  This 
  can 
  be 
  shown 
  with 
  an 
  apparatus 
  like 
  that 
  shown 
  

   in 
  fig. 
  7, 
  where 
  W 
  is 
  a 
  hot 
  AVehnelt 
  cathode, 
  connected 
  up 
  

  

  Tier, 
  7. 
  

  

  

  W 
  

  

  ©°'- 
  

  

  with 
  a 
  battery 
  of 
  small 
  storage-cells, 
  causing 
  it 
  to 
  emit 
  slow 
  

   cathode 
  rays 
  which 
  ionize 
  the 
  gas 
  in 
  the 
  tube 
  C, 
  a 
  and 
  h 
  are 
  

   two 
  carefully 
  insulated 
  Faraday 
  cylinders 
  ; 
  there 
  is 
  a 
  very 
  

   small 
  hole, 
  in 
  some 
  cases 
  less 
  tlian 
  -^ 
  mm. 
  in 
  diameter, 
  in 
  the 
  

   plate 
  d 
  which 
  separates 
  the 
  Faraday 
  cylinders 
  from 
  the 
  

   ionized 
  gas 
  ; 
  through 
  this 
  hole 
  the 
  ions 
  produced 
  in 
  C 
  can 
  

   diffuse 
  and 
  charge 
  up 
  the 
  Faraday 
  cylinders. 
  

  

  We 
  can, 
  however, 
  by 
  suitable 
  means 
  prevent 
  the 
  ions 
  

   passing 
  through 
  the 
  hole 
  : 
  the 
  negative 
  ones 
  which 
  are 
  

   mainly 
  corpuscles, 
  and 
  easily 
  deflected 
  by 
  a 
  magnet, 
  are 
  

   prevented 
  by 
  applying 
  a 
  strong 
  magnetic 
  field 
  which 
  bends 
  

   them 
  back 
  before 
  ihey 
  reach 
  the 
  cylinder 
  ; 
  the 
  positive 
  ions, 
  

   which 
  are 
  not 
  easily 
  deflected 
  by 
  a 
  magnet, 
  can 
  be 
  stopped 
  

   by 
  a 
  strong 
  electric 
  field 
  between 
  the 
  wire 
  gauze 
  / 
  and 
  the 
  

   top 
  of 
  the 
  cylinder 
  d, 
  the 
  gauze 
  being 
  negative 
  to 
  the 
  

   cylinder. 
  When 
  the 
  ions 
  are 
  stopped 
  by 
  these 
  means 
  we 
  find 
  

   that 
  though 
  no 
  perceptible 
  electric 
  charge 
  reaches 
  either 
  

   cylinder, 
  the 
  gas 
  between 
  them 
  is 
  a 
  conductor 
  of 
  electricity, 
  

   and 
  if 
  either 
  cylinder 
  is 
  charged 
  up 
  the 
  other 
  will 
  slowly 
  

   acquire 
  the 
  same 
  potential, 
  whether 
  that 
  potential 
  be 
  positive 
  

   or 
  negative. 
  

  

  That 
  the 
  conductivity 
  of 
  the 
  gas 
  is 
  not 
  due 
  to 
  ultra-violet 
  

   light 
  coming 
  from 
  the 
  luminous 
  discharge 
  in 
  the 
  upper 
  tube 
  is 
  

   shown 
  by 
  the 
  fact 
  that 
  it 
  is 
  destroyed 
  by 
  putting 
  a 
  thin 
  quartz 
  

   plate 
  over 
  the 
  hole 
  in 
  d 
  ; 
  and 
  since 
  it 
  is 
  also 
  destroyed 
  when 
  

   a 
  piece 
  of 
  the 
  thinnest 
  aluminium 
  foil 
  obtainable 
  is 
  placed 
  

  

  