﻿598 
  Prof. 
  Pollock 
  and 
  Mr. 
  Vonwiller 
  : 
  Experiments 
  

  

  The 
  Specific 
  Inductive 
  Capacity 
  of 
  a 
  Specimen 
  of 
  Glass. 
  

  

  It 
  has 
  been 
  shown 
  above 
  that, 
  with 
  the 
  bridge 
  in 
  a 
  fixed 
  

   position, 
  the 
  magnitude 
  of 
  the 
  readings 
  obtained 
  with 
  the 
  

   detector 
  depend 
  on 
  the 
  period 
  of 
  the 
  oscillation 
  of 
  the 
  con- 
  

   denser 
  circuit. 
  Keeping 
  the 
  inductance 
  of 
  the 
  circuit 
  

   constant, 
  under 
  certain 
  conditions, 
  the 
  deflexions 
  alter 
  rapidly 
  

   with 
  change 
  of 
  capacity. 
  The 
  proper 
  relation 
  between 
  the 
  

   period 
  of 
  the 
  condenser 
  discharge 
  and 
  the 
  length 
  of 
  the 
  wires 
  

   for 
  most 
  rapid 
  variation 
  of 
  deflexion 
  with 
  respect 
  to 
  change 
  

   of 
  capacity, 
  is 
  readily 
  found 
  for 
  any 
  set 
  of 
  circumstances. 
  

   The 
  conditions 
  must 
  be 
  such 
  as 
  to 
  give 
  a 
  relation 
  between 
  

   deflexion 
  and 
  distance 
  apart 
  of 
  condenser-plates 
  similar 
  to 
  

   that 
  represented 
  by 
  the 
  steep 
  sides 
  of 
  the 
  curve 
  in 
  fig. 
  3. 
  

  

  Advantage 
  has 
  been 
  taken 
  of 
  this 
  fact 
  of 
  rapid 
  change 
  of 
  

   deflexion 
  with 
  respect 
  to 
  change 
  of 
  capacity, 
  to 
  determine 
  the 
  

   specific 
  inductive 
  capacity 
  of 
  a 
  piece 
  of 
  glass. 
  

  

  All 
  the 
  specimens 
  of 
  glass 
  which 
  have 
  been 
  used 
  are 
  pieces 
  

   of 
  " 
  thick 
  polished 
  plate 
  glass 
  " 
  made 
  by 
  the 
  Compagnie 
  de 
  

   FlorefFe, 
  FlorefFe, 
  Belgium. 
  They 
  were 
  cut 
  at 
  the 
  same 
  time 
  

   from 
  one 
  sheet. 
  The 
  indices 
  of 
  refraction 
  at 
  15° 
  C, 
  as 
  

   determined 
  from 
  observations 
  of 
  the 
  angles 
  of 
  minimum 
  

   deviation 
  with 
  a 
  prism 
  of 
  the 
  material, 
  are 
  given 
  in 
  the 
  

   following 
  table 
  : 
  — 
  

  

  B 
  1-5283, 
  C 
  1-5296, 
  D 
  1-5320, 
  E 
  1-5355, 
  

   6,1-5361, 
  F 
  1-5386, 
  G 
  1*5446. 
  

  

  The 
  density 
  of 
  the 
  glass 
  at 
  23° 
  C. 
  is 
  2'54. 
  

  

  Care 
  has 
  been 
  taken 
  in 
  every 
  case 
  to 
  have 
  the 
  glass 
  clean 
  

   and 
  dry. 
  

  

  The 
  scheme 
  of 
  the 
  arrangement 
  by 
  which 
  the 
  specific 
  

   inductive 
  capacity 
  of 
  the 
  glass 
  has 
  been 
  determined 
  is 
  shown 
  

   in 
  fig. 
  5. 
  

  

  Each 
  of 
  the 
  wires 
  c 
  d 
  was 
  4*94 
  metres 
  total 
  length, 
  4*675 
  m. 
  

   straight, 
  and 
  0*265 
  m. 
  bent 
  round 
  at 
  condenser 
  end. 
  The 
  

   detector 
  was 
  placed 
  in 
  the 
  primary 
  circuit 
  30 
  cms. 
  from 
  c 
  c, 
  

   the 
  bridge 
  being 
  1*10 
  m. 
  from 
  the 
  same 
  line. 
  Preliminary 
  

   trials 
  had 
  shown 
  that 
  the 
  capacity 
  of 
  the 
  condenser 
  with 
  its 
  

   plates 
  separated 
  by 
  0*6 
  cm., 
  air 
  being 
  the 
  dielectric, 
  was 
  

   approximately 
  equal 
  to 
  its 
  capacity 
  when- 
  the 
  glass 
  was 
  

  

  