﻿224 
  J. 
  Trowbridge 
  — 
  Electrical 
  Oscillations 
  on 
  Iron 
  Wires. 
  

  

  the 
  mass 
  of 
  the 
  iron 
  magnetized. 
  Even 
  if 
  these 
  instantaneous 
  

   discharges 
  are 
  capable 
  of 
  magnetizing 
  iron, 
  .... 
  the 
  electro- 
  

   motive 
  impulses 
  or 
  sudden 
  rushes 
  of 
  electricity 
  do 
  not 
  mag- 
  

   netize 
  the 
  iron, 
  and 
  hence 
  do 
  not 
  find 
  in 
  it 
  any 
  greater 
  self- 
  

   inductive 
  opposition 
  than 
  they 
  would 
  find 
  in 
  a 
  non-magnetic 
  

   but 
  otherwise 
  similar 
  conductor. 
  Dr. 
  Lodge's 
  further 
  researches 
  

   seem 
  to 
  show 
  that 
  there 
  is 
  a 
  real 
  advantage 
  in 
  using 
  iron 
  for 
  

   lightning 
  conductors 
  over 
  copper, 
  and 
  that 
  its 
  greater 
  specific 
  

   resistance 
  and 
  higher 
  fusing 
  point 
  enables 
  an 
  iron 
  rod 
  or 
  tape 
  

   to 
  get 
  rid 
  safely 
  of 
  an 
  amount 
  of 
  electric 
  energy 
  stored 
  up 
  in 
  

   the 
  dielective 
  which 
  would 
  not 
  be 
  the 
  case 
  if 
  it 
  were 
  copper."* 
  

  

  Fleming 
  describes 
  in 
  full 
  Dr. 
  Lodge's 
  experiments 
  to 
  prove 
  

   the 
  non-magnetizability 
  of 
  iron 
  by 
  sudden 
  discharges 
  : 
  — 
  

  

  " 
  In 
  the 
  experiments 
  on 
  alternative 
  path, 
  as 
  described 
  by 
  Dr. 
  

   Lodge, 
  the 
  main 
  result 
  is 
  very 
  briefly 
  summed 
  up 
  by 
  saying 
  

   that, 
  when 
  a 
  sudden 
  discharge 
  had 
  to 
  pass 
  through 
  a 
  conductor, 
  

   it 
  was 
  found 
  that 
  iron 
  and 
  copper 
  acted 
  about 
  equally 
  well, 
  

   and 
  indeed 
  iron 
  sometimes 
  exhibited 
  a 
  little 
  superiority, 
  and 
  

   that 
  the 
  thickness 
  of 
  the 
  conductor 
  and 
  its 
  ordinary 
  conduc- 
  

   tivity 
  mattered 
  very 
  little 
  indeed. 
  ... 
  In 
  the 
  case 
  of 
  enorm- 
  

   ously 
  rapid 
  oscillations 
  the 
  value 
  of 
  the 
  impulsive 
  impedance 
  

   varies 
  in 
  simple 
  proportion 
  to 
  the 
  frequency 
  of 
  the 
  oscillations, 
  

   and 
  depends 
  on 
  the 
  form 
  and 
  size 
  of 
  the 
  circuit, 
  but 
  not 
  at 
  all 
  

   on 
  its 
  specific 
  resistance, 
  magnetic 
  permeability, 
  or 
  diameter. 
  . 
  

   .... 
  For 
  discharges 
  of 
  a 
  million 
  per 
  second 
  and 
  upwards, 
  

   such 
  as 
  occur 
  in 
  jar 
  discharges 
  and 
  perhaps 
  in 
  lightning, 
  the 
  

   impedance 
  of 
  all 
  reasonably 
  conducting 
  circuits 
  is 
  the 
  same, 
  

   and 
  independent 
  of 
  conductivity 
  and 
  permeability, 
  and 
  hardly 
  

   affected 
  by 
  enormous 
  changes 
  in 
  diameter."f 
  

  

  Turning 
  now 
  to 
  the 
  observations 
  of 
  Hertz, 
  we 
  find 
  it 
  stated 
  

   that 
  the 
  material, 
  the 
  resistance, 
  and 
  the 
  diameter 
  of 
  the 
  wire 
  

   of 
  the 
  micrometer 
  circuit 
  employed 
  by 
  him, 
  have 
  very 
  little 
  

   influence 
  on 
  the 
  result. 
  The 
  rate 
  of 
  propagation 
  of 
  an 
  electri- 
  

   cal 
  disturbance 
  along 
  a 
  conductor 
  depends 
  mainly 
  on 
  its 
  

   capacity 
  and 
  coefficient 
  of 
  self-induction, 
  and 
  only 
  to 
  a 
  small 
  

   extent 
  on 
  its 
  resistance. 
  Hertz 
  concludes 
  that, 
  owing 
  to 
  the 
  

   great 
  rapidity 
  of 
  the 
  alternations, 
  the 
  magnetism 
  of 
  the 
  iron 
  is 
  

   unable 
  to 
  follow 
  them, 
  and 
  therefore 
  has 
  no 
  effect 
  on 
  the 
  self- 
  

   induction. 
  When 
  a 
  portion 
  of 
  the 
  micrometer 
  circuit 
  em- 
  

   ployed 
  by 
  Hertz 
  was 
  surrounded 
  by 
  an 
  iron 
  tube, 
  or 
  replaced 
  

   by 
  an 
  iron 
  wire, 
  no 
  perceptible 
  effect 
  was 
  obtained, 
  and 
  thus 
  

   the 
  result 
  was 
  apparently 
  confirmed 
  that 
  the 
  magnetism 
  of 
  the 
  

   iron 
  is 
  unable 
  to 
  follow 
  such 
  rapid 
  oscillations, 
  and 
  therefore 
  

   exerts 
  no 
  appreciable 
  effect. 
  The 
  velocity 
  of 
  propagation 
  in 
  a 
  

   wire 
  has 
  a 
  definite 
  value 
  independent 
  of 
  its 
  dimensions 
  and 
  

   material. 
  Even 
  iron 
  wires 
  offer 
  no 
  exception 
  to 
  this, 
  showing 
  

  

  * 
  Fleming, 
  Induction 
  of 
  Electric 
  Currents, 
  p. 
  398. 
  f 
  Ibid, 
  p. 
  411. 
  

  

  