﻿602 
  Prof. 
  Pollock 
  and 
  Mr. 
  Von 
  wilier 
  : 
  Experiments 
  

  

  value 
  of 
  a, 
  given 
  in 
  Tables 
  III., 
  IV., 
  and 
  V. 
  3 
  the 
  results 
  

   are 
  : 
  — 
  

  

  7-85 
  

   7-80 
  

   7-95 
  

  

  Mean 
  7'87 
  at 
  26°'0 
  C. 
  

  

  The 
  specific 
  inductive 
  capacity 
  of 
  the 
  same 
  disk 
  of 
  glass 
  has 
  

   been 
  measured 
  by 
  an 
  absolute 
  electrometer 
  method 
  with 
  

   alternating 
  potential 
  -differences 
  having 
  a 
  frequency 
  of 
  about 
  

   50 
  per 
  second, 
  the 
  details 
  of 
  which 
  are 
  given 
  later. 
  The 
  

   results 
  so 
  obtained 
  for 
  K 
  are 
  : 
  — 
  

  

  Mean 
  7*97 
  at 
  25°\5 
  C. 
  

  

  The 
  evidence 
  afforded 
  by 
  this 
  investigation 
  is 
  not 
  sufficient 
  to 
  

   show 
  that 
  there 
  is 
  any 
  difference 
  between 
  the 
  specific 
  inductive 
  

   capacity 
  of 
  the 
  piece 
  of 
  glass 
  used 
  when 
  acted 
  on 
  by 
  alter- 
  

   nating 
  electric 
  forces 
  with 
  a 
  frequency 
  of 
  20 
  million 
  per 
  sec, 
  

   and 
  that 
  found 
  by 
  the 
  aid 
  of 
  forces 
  having 
  a 
  frequency 
  of 
  

   only 
  50 
  per 
  sec. 
  

  

  Lecher, 
  in 
  determining 
  the 
  specific 
  inductive 
  capacity 
  of 
  

   various 
  dielectrics 
  (Wied. 
  Ann. 
  xlii. 
  p. 
  150, 
  1891), 
  joins 
  the 
  

   ends 
  of 
  the 
  secondary 
  wires 
  to 
  condenser-plates. 
  Placing 
  a 
  

   slab 
  of 
  the 
  dielectric 
  between 
  the 
  plates, 
  he 
  finds 
  a 
  position 
  of 
  

   the 
  bridge 
  such 
  that 
  the 
  two 
  circuits 
  into 
  which 
  it 
  divides 
  the 
  

   wire 
  system 
  are 
  in 
  resonance. 
  This 
  position 
  depends 
  on 
  the 
  

   capacitv 
  of 
  the 
  end 
  condenser. 
  With 
  air 
  between 
  the 
  con- 
  

   denser-plates, 
  he 
  then 
  determines 
  for 
  various 
  capacities 
  the 
  

   corresponding 
  positions 
  of 
  the 
  bridge, 
  and 
  by 
  interpolation 
  

   finds 
  the 
  capacity 
  which 
  would 
  give 
  the 
  same 
  position 
  of 
  the 
  

   bridge 
  as 
  in 
  the 
  former 
  case. 
  This 
  capacity 
  is 
  then 
  equal 
  

   to 
  that 
  of 
  the 
  condenser 
  with 
  the 
  material 
  under 
  investiga- 
  

   tion 
  as 
  the 
  dielectric. 
  Cohn 
  and 
  Heer-wagen 
  (Wied. 
  Ann.- 
  

   xliii. 
  p. 
  343, 
  1891), 
  and 
  Thomson 
  (Recent 
  Researches, 
  p. 
  340) 
  

   give 
  the 
  formula 
  applicable 
  to 
  Lecher's 
  end 
  circuit. 
  The 
  

   formula 
  may 
  be 
  written 
  : 
  — 
  

  

  ,2tts 
  2ttG 
  

   cot—- 
  = 
  — 
  -, 
  

   X 
  X 
  c 
  

  

  s 
  being 
  the 
  length 
  of 
  the 
  wire 
  circuit 
  of 
  the 
  secondary 
  system 
  

   when 
  resonance 
  obtains, 
  X 
  the 
  wave-length 
  of 
  the 
  vibration 
  

  

  