﻿254 
  Mr. 
  B. 
  Hodgson 
  on 
  the 
  Conductivity 
  of 
  

  

  To 
  prevent 
  leak 
  across 
  the 
  ebonite 
  supports 
  the 
  following 
  

   arrangement 
  was 
  employed. 
  The 
  electrode 
  B 
  (fig. 
  2) 
  was 
  

  

  Fiff. 
  2. 
  

  

  supported 
  by 
  ebonite 
  stems 
  N 
  N, 
  and 
  G 
  by 
  similar 
  stems 
  PP, 
  

   from 
  an 
  earthed 
  plate 
  Q. 
  This 
  avoided 
  effectively 
  any 
  

   spurious 
  effect 
  due 
  to 
  defective 
  insulation. 
  

  

  The 
  radium 
  used 
  contained 
  about 
  5 
  mg. 
  of 
  pure 
  radium 
  

   bromide 
  and 
  was 
  sealed 
  in 
  a 
  glass 
  vessel 
  with 
  a 
  thin 
  mica 
  

   cover. 
  When 
  acting 
  on 
  the 
  dielectric 
  it 
  was 
  placed 
  in 
  the 
  

   lead 
  box 
  L 
  and 
  the 
  rays 
  passed 
  through 
  2 
  cm. 
  of 
  liquid 
  before 
  

   reaching 
  that 
  between 
  the 
  electrodes. 
  

  

  Ki 
  (fig. 
  1) 
  was 
  a 
  key 
  which 
  enabled 
  the 
  electrode 
  C 
  and 
  

   electrometer 
  to 
  be 
  *' 
  earthed" 
  or 
  insulated 
  at 
  will. 
  

  

  G 
  W'as 
  a 
  capacity 
  added 
  when 
  necessary 
  to 
  lessen 
  the 
  rate 
  

   of 
  movement 
  of 
  needle. 
  To 
  prevent 
  any 
  leak 
  due 
  to 
  ionized 
  

   air 
  the 
  following 
  arrangement 
  was 
  used. 
  The 
  wire 
  E 
  was 
  

   passed 
  through 
  a 
  glass 
  tube 
  T 
  covered 
  with 
  tinfoil 
  and 
  

   earthed, 
  and 
  tilled 
  with 
  heavy 
  paraffin 
  oil 
  of 
  high 
  insulation. 
  

   By 
  this 
  means 
  air 
  was 
  removed 
  from 
  all 
  connexions 
  right 
  up 
  

   to 
  the 
  electrometer, 
  and 
  so 
  chances 
  of 
  leak 
  due 
  to 
  air-ioniza- 
  

   tion 
  were 
  eliminated. 
  This 
  oil 
  proved 
  a 
  splendid 
  insulator, 
  

   and 
  when 
  the 
  system 
  was 
  charged 
  up 
  to 
  1 
  volt 
  no 
  leak 
  could 
  

   be 
  detected 
  in 
  6 
  min. 
  

  

  The 
  electrometer, 
  keys 
  and 
  condenser 
  were 
  enclosed 
  in 
  

   earthed 
  tin 
  boxes 
  M, 
  M. 
  R, 
  R 
  was 
  a 
  large 
  earthed 
  wire 
  

   cage. 
  In 
  performing 
  an 
  experiment 
  the 
  electrode 
  B 
  was 
  

   connected 
  to 
  a 
  set 
  of 
  storage-cells 
  and 
  the 
  key 
  K 
  was 
  lifted, 
  

   insulating 
  the 
  electrode 
  0. 
  The 
  needle 
  then 
  began 
  to 
  

   move, 
  and 
  scale 
  readings 
  were 
  taken 
  every 
  \ 
  minute. 
  A 
  

   knowledge 
  of 
  the 
  capacity 
  of 
  the 
  system 
  and 
  the 
  sensitiveness 
  

  

  