﻿Longden 
  — 
  Electrical 
  Resistance 
  of 
  Thin 
  Films. 
  107 
  

  

  Art. 
  XL. 
  — 
  Electrical 
  Resistance 
  of 
  Thin 
  Films 
  Deposited 
  

   by 
  Cathode 
  Discharge 
  ; 
  by 
  A. 
  C. 
  Longden. 
  

  

  More 
  than 
  twenty 
  years 
  ago, 
  a 
  method 
  of 
  depositing 
  metal- 
  

   lic 
  films 
  upon 
  glass, 
  b} 
  T 
  means 
  of 
  electrical 
  discharge 
  in 
  

   exhausted 
  tubes, 
  was 
  devised 
  by 
  Professor 
  A. 
  W. 
  Wright* 
  of 
  

   Yale 
  University. 
  This 
  physicist 
  not 
  only 
  described 
  a 
  method 
  

   of 
  producing 
  the 
  films, 
  but 
  he 
  pointed 
  out 
  the 
  peculiar 
  bril- 
  

   liancy 
  of 
  their 
  mirror 
  surfaces 
  and 
  their 
  colors 
  by 
  transmitted 
  

   light. 
  He 
  also 
  noted 
  the 
  rate 
  of 
  deposition 
  of 
  the 
  different 
  

   metals, 
  and 
  suggested 
  that 
  as 
  aluminum 
  and 
  magnesium 
  were 
  

   deposited 
  very 
  slowly, 
  these 
  metals 
  should 
  be 
  used 
  as 
  elec- 
  

   trodes 
  in 
  vacuum 
  tubes, 
  in 
  order 
  to 
  prevent 
  the 
  discoloration 
  

   of 
  the 
  glass 
  in 
  the 
  neighborhood 
  of 
  the 
  cathode, 
  which 
  was 
  so 
  

   common 
  when 
  platinum 
  electrodes 
  were 
  employed. 
  

  

  During 
  the 
  past 
  two 
  years 
  I 
  have 
  been 
  investigating 
  the 
  

   electrical 
  resistance 
  of 
  thin 
  films 
  deposited 
  by 
  a 
  modification 
  

   of 
  Professor 
  Wright's 
  process. 
  

  

  Quite 
  early 
  in 
  this 
  investigation 
  it 
  was 
  observed 
  that 
  very 
  

   thin 
  films 
  have 
  enormously 
  high 
  electrical 
  resistance, 
  — 
  very 
  

   much 
  higher, 
  as 
  compared 
  with 
  thicker 
  films 
  of 
  the 
  same 
  kind, 
  

   than 
  their 
  thickness 
  would 
  seem 
  to 
  indicate. 
  It 
  was 
  also 
  

   observed 
  that, 
  in 
  many 
  cases, 
  temperature 
  changes 
  did 
  not 
  pro- 
  

   duce 
  such 
  variations 
  in 
  electrical 
  resistance 
  as 
  are 
  common 
  in 
  

   ordinary 
  wire 
  resistances. 
  

  

  As 
  early 
  as 
  April 
  30, 
  1898, 
  careful 
  measurements 
  of 
  the 
  

   resistance 
  of 
  a 
  particular 
  platinum 
  film 
  at 
  different 
  tempera- 
  

   tures, 
  revealed 
  the 
  fact 
  that 
  its 
  temperature-coefficient 
  was 
  

   negative, 
  its 
  value 
  being 
  —0-00013. 
  Numerous 
  measurements 
  

   which 
  have 
  been 
  made 
  since 
  that 
  time, 
  fully 
  warrant 
  the 
  state- 
  

   ment 
  that 
  all 
  very 
  thin 
  films 
  have 
  negative 
  temperature-coeffi- 
  

   cients, 
  and 
  that 
  films 
  within 
  a 
  certain 
  range 
  of 
  thickness 
  have 
  

   zero 
  or 
  negligible 
  temperature-coefficients. 
  

  

  A 
  peculiar 
  relation 
  is 
  found 
  to 
  exist 
  between 
  the 
  magnitude 
  

   of 
  the 
  temperature-coefficient 
  of 
  a 
  film 
  resistance, 
  and 
  the 
  

   process 
  of 
  artificial 
  ageing. 
  If 
  a 
  film 
  be 
  placed 
  in 
  a 
  bath 
  of 
  

   melted 
  paraffin 
  or 
  hot 
  oil, 
  and 
  measured 
  while 
  in 
  the 
  bath, 
  its 
  

   resistance 
  is 
  usually 
  found 
  to 
  change 
  quite 
  rapidly 
  at 
  first, 
  and 
  

   then 
  gradually 
  less 
  rapidly 
  for 
  a 
  considerable 
  length 
  of 
  time. 
  

   If 
  the 
  first 
  rapid 
  change 
  is 
  an 
  increase, 
  a 
  maximum 
  is 
  soon 
  

   reached, 
  and 
  then 
  follows 
  a 
  gradual 
  and 
  long 
  continued 
  

   decrease. 
  In 
  such 
  a 
  case, 
  the 
  temperature-coefficient 
  is 
  positive. 
  

   If, 
  however, 
  the 
  first 
  change 
  is 
  a 
  decrease, 
  the 
  later 
  and 
  more 
  

   gradual 
  change 
  is 
  an 
  increase. 
  In 
  such 
  a 
  case 
  as 
  this, 
  the 
  

  

  * 
  This 
  Journal, 
  vol. 
  xiii, 
  pp. 
  49-55, 
  1877. 
  Ibid., 
  vol. 
  xiv, 
  pp. 
  169-178. 
  

  

  