﻿604 
  Prof. 
  Pollock 
  and 
  Mr. 
  Vonwiller 
  : 
  Experiments 
  

  

  the 
  glass 
  plate 
  less 
  in 
  diameter 
  than 
  the 
  small 
  plates, 
  and 
  in 
  

   contact 
  with 
  one 
  o£ 
  them 
  and 
  the 
  middle 
  plate. 
  It 
  is 
  doubtful, 
  

   however, 
  if 
  the 
  method 
  is 
  worth 
  considering, 
  on 
  account 
  of 
  the 
  

   difficulty 
  of 
  calculating 
  the 
  disturbance 
  in 
  the 
  field 
  caused 
  by 
  

   the 
  small 
  plates. 
  

  

  Blondlot 
  (Comptes 
  Rendus, 
  May 
  11th, 
  1891, 
  p. 
  1058) 
  

   compares 
  the 
  specific 
  inductive 
  capacity 
  of 
  a 
  piece 
  of 
  glass 
  

   with 
  that 
  of 
  sulphur 
  in 
  a 
  rapidly 
  changing 
  field. 
  The 
  way 
  

   in 
  which 
  the 
  value 
  of 
  K 
  is 
  calculated 
  is 
  not 
  shown, 
  but 
  from 
  

   the 
  value 
  given 
  it 
  appears 
  to 
  have 
  been 
  assumed 
  that 
  the 
  

   formula 
  for 
  plates 
  and 
  dielectric 
  infinite 
  in 
  extent, 
  the 
  

   dielectric 
  slabs 
  completely 
  filling 
  the 
  space 
  between 
  the 
  plates, 
  

   is 
  applicable 
  to 
  the 
  method. 
  This 
  is 
  very 
  far 
  from 
  being 
  the 
  

   case, 
  and 
  the 
  value 
  of 
  K, 
  2'9, 
  deduced 
  from 
  the 
  experiments 
  

   must 
  be 
  taken 
  as 
  incorrect. 
  

  

  Details 
  of 
  Absolute 
  Electrometer 
  Method. 
  

  

  The 
  apparatus 
  consists 
  of 
  a 
  brass 
  ring 
  and 
  a 
  circular 
  brass 
  

   plate, 
  each 
  1 
  cm. 
  thick 
  and 
  23 
  cms. 
  in 
  external 
  diameter. 
  They 
  

   are 
  placed 
  one 
  above 
  the 
  other, 
  being 
  separated 
  by 
  three 
  hard- 
  

   rubber 
  distance-pieces 
  of 
  any 
  required 
  length. 
  The 
  ring 
  

   forms 
  a 
  guard-ring 
  to 
  a 
  movable 
  plate 
  attached 
  to 
  the 
  arm 
  of 
  

   a 
  balance. 
  The 
  diameter 
  of 
  the 
  swinging 
  plate 
  is 
  8*95 
  cms., 
  

   that 
  of 
  the 
  aperture 
  in 
  the 
  ring 
  being 
  9*05 
  cms. 
  The 
  

   lower 
  plate 
  is 
  provided 
  with 
  three 
  levelling-screws, 
  which 
  

   enable 
  the 
  final 
  adjustment 
  for 
  parallelism 
  to 
  be 
  made 
  with 
  

   great 
  accuracy. 
  The 
  opposed 
  surfaces 
  of 
  the 
  plates 
  are 
  

   carefully 
  scraped 
  plane. 
  The 
  plates 
  are 
  webbed 
  across 
  their 
  

   outer 
  surfaces, 
  being 
  purposely 
  made 
  very 
  massive 
  to 
  avoid 
  

   any 
  uncertainty 
  in 
  the 
  distance 
  measurements 
  due 
  to 
  bending. 
  

  

  Distance 
  measurements. 
  — 
  The 
  distance 
  measurements 
  are 
  

   made 
  by 
  a 
  specially 
  constructed 
  calipers 
  which 
  rests 
  on 
  the 
  

   lower 
  plate, 
  and 
  which 
  may 
  be 
  placed 
  to 
  measure 
  the 
  distance 
  

   between 
  the 
  plates 
  at 
  any 
  point. 
  The 
  lever 
  is 
  moved 
  by 
  a 
  

   micrometer-screw 
  outside 
  the 
  plates. 
  The 
  head 
  of 
  the 
  screw 
  

   is 
  3 
  cms. 
  in 
  diameter, 
  and 
  is 
  divided 
  into 
  25 
  divisions, 
  the 
  

   value 
  of 
  each 
  division 
  being 
  0*023 
  mm. 
  The 
  arrester 
  of 
  the 
  

   balance 
  is 
  altered 
  so 
  that 
  when 
  the 
  beam 
  is 
  resting 
  on 
  its 
  

   knife-edge, 
  the 
  mass 
  in 
  the 
  pan 
  being 
  greater 
  than 
  that 
  

   necessary 
  to 
  counterbalance 
  the 
  swinging 
  plate, 
  the 
  beam 
  can 
  

   be 
  tilted 
  until 
  the 
  pointer 
  is 
  exactly 
  at 
  the 
  middle 
  mark 
  of 
  the 
  

   scale. 
  When 
  the 
  pointer 
  is 
  in 
  this 
  position, 
  distance 
  measure- 
  

   ments 
  are 
  taken 
  at 
  three 
  equidistant 
  points 
  near 
  the 
  edge 
  of 
  

   the 
  swinging 
  plate. 
  A 
  full 
  set 
  of 
  the 
  most 
  discordant 
  readings 
  

   is 
  given 
  here 
  to 
  show 
  the 
  accuracy 
  attained. 
  The 
  readings 
  

   are 
  in 
  terms 
  of 
  divisions 
  of 
  the 
  micrometer-head, 
  the 
  revolution 
  

  

  