﻿560 
  Prof. 
  J. 
  S. 
  Townsend 
  on 
  the 
  Conductivity 
  

  

  Metallic 
  connexion 
  was 
  made 
  with 
  the 
  zinc 
  plate 
  by 
  a 
  rod 
  

   passing 
  through 
  the 
  side-tube 
  T. 
  Tire 
  rod 
  entered 
  the 
  ap- 
  

   paratus 
  through 
  an 
  ebonite 
  plug 
  in 
  the 
  end 
  of 
  the 
  side-tube 
  , 
  

   and 
  was 
  thus 
  insulated 
  from 
  the 
  glass 
  tube. 
  

  

  The 
  zinc 
  plate 
  was 
  in 
  connexion 
  with 
  one 
  pair 
  of 
  quadrants 
  

   of 
  an 
  electrometer, 
  the 
  other 
  pair 
  of 
  quadrants 
  being 
  to 
  earth. 
  

   The 
  wire 
  leading 
  from 
  the 
  apparatus 
  to 
  the 
  insulated 
  quad- 
  

   rants 
  passed 
  along 
  the 
  centre 
  of 
  an 
  earth-connected 
  brass 
  

   tube 
  to 
  prevent 
  induction-effects. 
  The 
  silvered 
  plate 
  A 
  was 
  

   connected 
  to 
  the 
  positive 
  terminal 
  of 
  a 
  battery 
  of 
  small 
  

   Leclanche 
  cells, 
  and 
  the 
  other 
  terminal 
  was 
  put 
  to 
  earth. 
  

   When 
  it 
  was 
  required 
  to 
  make 
  experiments 
  with 
  a 
  constant 
  

   electric 
  force, 
  the 
  number 
  of 
  cells 
  employed 
  was 
  proportional 
  

   to 
  the 
  distance 
  between 
  the 
  plates. 
  The 
  cells 
  were 
  set 
  up 
  in 
  

   sets 
  of 
  24 
  ; 
  each 
  set 
  had 
  exactly 
  the 
  same 
  potential, 
  35 
  volts. 
  

  

  The 
  plate 
  B 
  connected 
  to 
  the 
  electrometer 
  never 
  deviated 
  

   much 
  from 
  zero 
  potential, 
  so 
  that 
  during 
  an 
  experiment 
  the 
  

   electric 
  force 
  between 
  the 
  plates 
  was 
  practically 
  constant 
  and 
  

   equal 
  to 
  the 
  potential 
  of 
  the 
  battery 
  divided 
  by 
  the 
  distance 
  

   between 
  the 
  plates. 
  

  

  Strips 
  of 
  tinfoil 
  were 
  wrapped 
  round 
  the 
  ebonite 
  tube 
  E 
  

   (about 
  halfway 
  between 
  the 
  brass 
  cap 
  and 
  the 
  disk 
  E), 
  and 
  

   connected 
  to 
  earth 
  through 
  a 
  second 
  side-tube 
  (not 
  shown 
  in 
  

   the 
  figure). 
  This 
  arrangement 
  prevented 
  any 
  leakage 
  of 
  

   electricity 
  over 
  the 
  surface 
  of 
  the 
  ebonite 
  from 
  the 
  plate 
  A 
  

   to 
  the 
  apparatus 
  connected 
  with 
  the 
  electrometer. 
  

  

  The 
  ultra-violet 
  light 
  was 
  produced 
  by 
  the 
  spark 
  of 
  a 
  

   leyden-jar 
  discharge 
  taking 
  place 
  between 
  two 
  aluminium 
  

   points. 
  An 
  induction-coil 
  was 
  used 
  for 
  charging 
  the 
  leyden- 
  

   jar. 
  The 
  whole 
  apparatus 
  for 
  producing 
  the 
  light 
  was 
  sur- 
  

   rounded 
  by 
  a 
  metal 
  screen 
  connected 
  to 
  earth, 
  so 
  that 
  the 
  

   sparking 
  should 
  have 
  no 
  direct 
  effect 
  on 
  the 
  electrometer. 
  

  

  The 
  spark-gap 
  was 
  at 
  the 
  focus 
  of 
  a 
  quartz 
  lens 
  placed 
  in 
  

   an 
  aperture 
  of 
  the 
  screen 
  vertically 
  above 
  the 
  parallel 
  plates 
  

   A 
  and 
  B. 
  A 
  lead 
  screen 
  with 
  a 
  circular 
  hole 
  one 
  centimetre 
  

   in 
  diameter 
  was 
  placed 
  between 
  the 
  lens 
  and 
  the 
  quartz 
  

   plate, 
  in 
  order 
  to 
  confine 
  the 
  light 
  to 
  the 
  central 
  portion 
  of 
  

   the 
  zinc 
  plate. 
  The 
  latter 
  was 
  2*4 
  centimetres 
  in 
  diameter, 
  

   so 
  that 
  when 
  the 
  light 
  fell 
  upon 
  it 
  the 
  conductivity 
  took 
  

   place 
  only 
  in 
  the 
  space 
  where 
  the 
  field 
  of 
  force 
  was 
  uniform. 
  

  

  The 
  apparatus 
  was 
  tested 
  at 
  various 
  times, 
  and 
  it 
  was 
  

   found 
  that 
  no 
  electrometer 
  deflexion 
  was 
  produced 
  by 
  the 
  

   sparking 
  when 
  the 
  aperture 
  in 
  the 
  lead 
  screen 
  was 
  closed. 
  

  

  3. 
  The 
  observations 
  were 
  made 
  in 
  the 
  usual 
  manner. 
  The 
  

   coil 
  which 
  charged 
  the 
  leyden-jar 
  was 
  turaed 
  on 
  for 
  a 
  fixed 
  

   time, 
  and 
  the 
  electrometer 
  deflexion 
  was 
  observed. 
  Several 
  

  

  