﻿Alternating- 
  Current 
  Spark 
  Potentials. 
  701 
  

  

  The 
  hioh-Yoltao^e 
  terminals 
  are 
  connected 
  with 
  the 
  dis- 
  

   charger, 
  represented 
  on 
  Plate 
  XIX. 
  This 
  piece 
  o£ 
  apparatus- 
  

   was 
  specially 
  constructed 
  for 
  this 
  series 
  o£ 
  experiments 
  by 
  

   the 
  Societe 
  Genevoise 
  pour 
  la 
  Construction 
  d''Instrument5 
  

   de 
  Physique, 
  accordino- 
  to 
  the 
  plans 
  furnished 
  by 
  us. 
  

  

  Of 
  the 
  two 
  marble 
  columns 
  carrying 
  the 
  electrodes 
  one 
  is 
  

   attached 
  to 
  a 
  block 
  of 
  ebonite, 
  which 
  in 
  turn 
  is 
  firmly 
  bolted 
  

   to 
  a 
  cast-irou 
  frame 
  ; 
  the 
  other 
  is 
  similarly 
  constructed, 
  but 
  

   is 
  bolted 
  to 
  a 
  rider, 
  capable 
  of 
  being 
  displaced 
  horizontally 
  

   by 
  means 
  of 
  a 
  threaded 
  screw. 
  This 
  screw 
  was 
  constructed 
  

   by 
  the 
  Societe 
  GeneA'oise 
  with 
  their 
  customary 
  care, 
  and 
  its 
  

   pitch 
  guaranteed 
  to 
  be 
  exact 
  to 
  the 
  hundredth 
  part 
  of 
  a 
  

   millimetre. 
  Parallel 
  to 
  the 
  screw 
  is 
  a 
  metre-scale 
  along 
  

   which 
  slides 
  a 
  little 
  brass 
  plate 
  attached 
  to 
  the 
  rider. 
  This 
  

   little 
  brass 
  plate 
  carries 
  a 
  fine 
  scratch, 
  enabling 
  one 
  to 
  read 
  

   off 
  on 
  the 
  metre 
  rod 
  the 
  number 
  of 
  entire 
  millimetres- 
  

   Attached 
  to 
  the 
  outer 
  end 
  of 
  the 
  screw 
  is 
  a 
  circular 
  disk 
  of 
  

   brass, 
  having 
  its 
  circumference 
  divided 
  into 
  100 
  equal 
  parts, 
  

   enabling 
  one 
  to 
  read 
  exactly 
  to 
  the 
  hundredth 
  part 
  of 
  a 
  

   millimetre. 
  Joined 
  to 
  the 
  screw 
  of 
  the 
  discharger, 
  by 
  means 
  

   of 
  a 
  ball 
  and 
  socket 
  joint, 
  is 
  a 
  cylinder 
  of 
  ebonite, 
  having 
  a 
  

   diameter 
  of 
  4 
  cm. 
  and 
  a 
  length 
  of 
  85 
  cm. 
  The 
  other 
  end 
  

   of 
  this 
  ebonite 
  rod 
  is 
  furnished 
  with 
  a 
  disk 
  for 
  measuring 
  

   exactly 
  to 
  the 
  hundredth 
  part 
  of 
  a 
  millimetre, 
  and 
  also 
  with 
  

   an 
  ebonite 
  crank 
  for 
  turning 
  the 
  screw. 
  This 
  appliance 
  

   enables 
  one 
  to 
  move 
  the 
  screw 
  and 
  take 
  the 
  exact 
  measure- 
  

   ments, 
  without 
  danger 
  to 
  the 
  operator 
  from 
  the 
  high 
  potential 
  

   of 
  the 
  discharger. 
  The 
  marble 
  columns 
  contain 
  two 
  metallic 
  

   cylinders 
  carefully 
  centred: 
  these 
  are 
  furnished 
  with 
  screws, 
  

   upon 
  which 
  the 
  various 
  spherical 
  electrodes 
  are 
  to 
  be 
  

   screwed. 
  

  

  Most 
  of 
  the 
  previous 
  determinations 
  of 
  spark 
  potentials 
  

   were 
  made 
  by 
  setting 
  the 
  electrodes 
  a 
  given 
  distance 
  apart, 
  

   and 
  then 
  raising 
  the 
  voltage 
  slowly 
  until 
  the 
  spark 
  jumped. 
  

   This 
  method 
  of 
  procedure 
  rendered 
  the 
  exact 
  reading 
  of 
  the 
  

   difference 
  of 
  potential 
  very 
  difficult 
  because 
  of 
  the 
  failure 
  of 
  

   the 
  instruments 
  in 
  responding 
  quickly 
  enough 
  to 
  the 
  change 
  

   of 
  potential 
  ; 
  thus 
  the 
  individual 
  readings 
  differed 
  greatly. 
  

   To 
  eliminate 
  this 
  source 
  of 
  error 
  we 
  proceeded 
  in 
  the 
  follow- 
  

   ing 
  manner. 
  The 
  ends 
  of 
  the 
  wires 
  leading 
  to 
  the 
  spherical 
  

   electrodes 
  were 
  joined 
  to 
  a 
  precision-milliamperemeter 
  of 
  

   Siemens 
  and 
  Halske 
  by 
  means 
  of 
  suitable 
  resistances. 
  One 
  

   of 
  the 
  electrodes 
  was 
  kept 
  grounded. 
  When 
  the 
  milliampere- 
  

   meter 
  readings 
  were 
  definite 
  and 
  constant 
  the 
  electrodes 
  were 
  

   brought 
  closer 
  by 
  moving 
  the 
  one 
  that 
  was 
  grounded. 
  In 
  

   this 
  wav 
  we 
  were 
  enabled 
  to 
  obtain 
  results 
  whose 
  maximum 
  

  

  