﻿426 
  Mr. 
  C. 
  A. 
  Chant 
  : 
  An 
  Experimental 
  Investigation 
  

  

  For 
  very, 
  rapid 
  oscillations 
  the 
  resistance 
  

  

  R'= 
  ^(ipZ/*E) 
  (I) 
  

  

  in 
  which 
  I 
  is 
  the 
  length 
  o£ 
  the 
  conductor, 
  //, 
  its 
  magnetic 
  

   permeability, 
  R 
  its 
  resistance 
  to 
  steady 
  currents, 
  and 
  p 
  = 
  2irn, 
  

   where 
  n 
  is 
  the 
  frequency. 
  

  

  In 
  1890 
  Stefan*, 
  in 
  a 
  paper 
  on 
  electric 
  oscillations 
  in 
  

   straight 
  conductors, 
  also 
  obtained 
  formulas 
  for 
  the 
  resistance 
  

   and 
  self-induction. 
  With 
  very 
  high 
  frequency 
  his 
  expression 
  

   for 
  the 
  resistance 
  is 
  

  

  R'=Rira\/^, 
  

  

  (2) 
  

  

  where 
  a 
  is 
  the 
  radius 
  of 
  the 
  conductor 
  and 
  a 
  its 
  specific 
  

   resistance. 
  This 
  formula 
  is 
  equivalent 
  to 
  that 
  given 
  by 
  

   Rayleigh. 
  He 
  remarks 
  that 
  for 
  very 
  great 
  frequencies 
  

   metallic 
  conductors 
  act 
  much 
  as 
  though 
  without 
  resistance, 
  

   but 
  electrolytes 
  behave 
  very 
  differently 
  on 
  account 
  or 
  their 
  

   very 
  high 
  resistance. 
  He 
  finds 
  that 
  for 
  a 
  cylindrical 
  copper 
  

   conductor 
  1 
  cm. 
  in 
  diameter, 
  with 
  a 
  frequency 
  of 
  50 
  millions, 
  

   the 
  current-density 
  at 
  a 
  depth 
  of 
  0"004 
  cm. 
  is 
  only 
  1/100 
  of 
  

   that 
  at 
  the 
  surface 
  ; 
  while 
  for 
  a 
  tube 
  of 
  equal 
  size 
  of 
  carbon 
  

   disulphide 
  the 
  current-density 
  at 
  the 
  centre 
  is 
  but 
  0*8 
  per 
  

   cent, 
  lower 
  than 
  at 
  the 
  surface, 
  — 
  in 
  other 
  words, 
  the 
  current 
  

   is 
  practically 
  uniform. 
  

  

  If, 
  now, 
  the 
  action 
  enters 
  the 
  conductor 
  from 
  the 
  surround- 
  

   ing 
  dielectric 
  and 
  is 
  prevented 
  from 
  penetrating 
  very 
  far 
  by 
  

   the 
  rapidity 
  of 
  the 
  oscillations, 
  it 
  is 
  evident 
  that 
  very 
  thin 
  

   layers 
  of 
  metal 
  should 
  be 
  sufficient 
  to 
  ward 
  off 
  electrical 
  

   undulations, 
  either 
  by 
  absorption 
  or 
  reflexion.. 
  

  

  In 
  a 
  paper 
  published 
  in 
  1889, 
  Hertz 
  f 
  described 
  experi- 
  

   ments 
  made 
  to 
  find 
  out 
  how 
  thick 
  a 
  metallic 
  film 
  was 
  needed 
  

   to 
  screen 
  from 
  his 
  rapid 
  oscillations. 
  Tinfoil, 
  Dutch 
  metal, 
  

   and 
  gilt 
  paper 
  acted 
  perfectly. 
  The 
  thickness 
  of 
  the 
  metal 
  

   on 
  the 
  latter 
  he 
  estimated 
  at 
  1/20 
  mm., 
  though 
  it 
  was 
  probably 
  

   much 
  less 
  than 
  that 
  amount. 
  Chemically 
  deposited 
  silver 
  

   failed 
  when 
  the 
  film 
  was 
  so 
  thin 
  as 
  not 
  to 
  be 
  opaque 
  to 
  light. 
  

   The 
  thickness 
  of 
  the 
  film 
  he 
  places 
  at 
  less 
  than 
  1/1000 
  mm. 
  

   It 
  was 
  probably 
  not 
  1/10 
  of 
  that 
  thickness 
  and, 
  moreover, 
  

   hardly 
  continuous 
  metal. 
  He 
  remarks 
  that 
  the 
  action 
  of 
  the 
  

   waves 
  scarcely 
  penetrates 
  farther 
  into 
  the 
  wire 
  than 
  does 
  the 
  

   light 
  which 
  is 
  reflected 
  from 
  its 
  surface. 
  Similar 
  experi- 
  

   ments 
  on 
  the 
  screening 
  effect 
  of 
  extremely 
  thin 
  metal 
  leaf 
  

   are 
  given 
  by 
  Lodge 
  and 
  others. 
  

  

  * 
  Wied. 
  Ann. 
  xli. 
  p. 
  400 
  (1890). 
  

   t 
  'Electric 
  Waves,' 
  p. 
  160. 
  

  

  