﻿408 
  Longden 
  — 
  Electrical 
  Resistance 
  of 
  Thin 
  Films 
  

  

  temperature-coefficient 
  is 
  negative. 
  If 
  the 
  resistance 
  does 
  not 
  

   change 
  at 
  first, 
  it 
  will 
  not 
  change 
  at 
  all. 
  In 
  such 
  a 
  case 
  the 
  

   temperature-coefficient 
  is 
  of 
  course 
  zero. 
  A 
  small 
  temperature- 
  

   coefficient 
  is 
  therefore 
  desirable, 
  not 
  only 
  for 
  its 
  own 
  sake, 
  but 
  

   because 
  a 
  film 
  resistance 
  having 
  a 
  small 
  temperature- 
  coefficient 
  

   does 
  not 
  require 
  much 
  artificial 
  ageing 
  ; 
  and 
  because 
  such 
  a 
  

   resistance 
  is 
  exceedingly 
  reliable 
  after 
  the 
  ageing 
  process 
  is 
  

   finished. 
  

  

  Film 
  resistances, 
  after 
  artificial 
  ageing, 
  should 
  be 
  protected 
  

   from 
  the 
  atmosphere. 
  The 
  necessary 
  protection 
  may 
  be 
  

   afforded 
  in 
  any 
  one 
  of 
  a 
  variety 
  of 
  ways. 
  The 
  film 
  may 
  be 
  

   sealed 
  in 
  a 
  vacuum 
  tube, 
  it 
  may 
  be 
  embedded 
  in 
  paraffin, 
  or 
  it 
  

   ma} 
  r 
  be 
  coated 
  with 
  a 
  varnish 
  prepared 
  by 
  dissolving 
  india- 
  

   rubber 
  in 
  carbon 
  disnlphide. 
  

  

  In 
  the 
  early 
  part 
  of 
  this 
  investigation, 
  the 
  electrical 
  contacts 
  

   between 
  the 
  films 
  and 
  their 
  terminal 
  wires 
  gave 
  no 
  small 
  

   amount 
  of 
  trouble. 
  Clamped 
  contacts 
  were 
  unsatisfactory, 
  

   both 
  on 
  account 
  of 
  the 
  delicacy 
  of 
  some 
  of 
  the 
  very 
  thin 
  films, 
  

   and 
  on 
  account 
  of 
  the 
  tendency 
  of 
  the 
  oil 
  or 
  paraffin 
  to 
  force 
  

   its 
  way 
  into 
  the 
  joints 
  during 
  the 
  process 
  of 
  artificial 
  ageing. 
  

  

  To 
  overcome 
  these 
  difficulties, 
  several 
  methods 
  of 
  making 
  

   electrical 
  connections 
  were 
  devised. 
  The 
  one 
  which 
  was 
  most 
  

   used, 
  consisted 
  in 
  depositing 
  very 
  thick 
  films 
  of 
  negligible 
  

   resistance 
  upon 
  the 
  ends 
  of 
  the 
  thin 
  resistance 
  films. 
  Terminal 
  

   wires 
  were 
  then 
  wound 
  on, 
  and 
  the 
  joints 
  between 
  the 
  wires 
  

   and 
  the 
  thick 
  ends 
  of 
  the 
  films 
  were 
  permanently 
  secured 
  by 
  

   the 
  electrolytic 
  deposition 
  of 
  copper 
  and 
  silver 
  upon 
  them. 
  

  

  These 
  resistances, 
  when 
  properly 
  prepared, 
  are 
  not 
  to 
  be 
  

   classed 
  with 
  carbon 
  resistances, 
  but 
  may 
  be 
  regarded 
  as 
  stand- 
  

   ards 
  of 
  a 
  very 
  high 
  degree 
  of 
  accuracy. 
  They 
  may 
  have 
  any 
  

   desired 
  value, 
  from 
  a 
  few 
  ohms 
  up 
  to 
  several 
  megohms. 
  They 
  

   may 
  be 
  made 
  of 
  pure 
  metals 
  and 
  thus 
  be 
  free 
  from 
  the 
  danger 
  

   of 
  disintegration 
  which 
  is 
  so 
  common 
  in 
  alloys 
  ; 
  and 
  the 
  metals 
  

   chosen 
  may 
  be 
  such 
  as 
  are 
  least 
  affected 
  by 
  external 
  influences. 
  

   In 
  addition 
  to 
  these 
  valued 
  qualities 
  as 
  pure 
  metals, 
  they 
  pos- 
  

   sess 
  the 
  only 
  advantages 
  of 
  alloys, 
  namely, 
  high 
  resistance 
  and 
  

   low 
  temperature-coefficients. 
  After 
  suitable 
  artificial 
  ageing 
  

   and 
  subsequent 
  protection 
  from 
  the 
  atmosphere, 
  they 
  may 
  be 
  

   regarded 
  as 
  perfectly 
  trustworthy 
  standards. 
  

  

  It 
  is 
  important, 
  however, 
  that 
  the 
  artificial 
  ageing 
  process 
  

   should 
  be 
  thorough. 
  Otherwise 
  the 
  gradual 
  change 
  in 
  resist- 
  

   ance 
  which 
  the 
  films 
  are 
  undergoing 
  may 
  continue 
  for 
  several 
  

   months. 
  

  

  I 
  append 
  a 
  few 
  records 
  for 
  the 
  purpose 
  of 
  presenting 
  the 
  

   results 
  of 
  perfect 
  and 
  imperfect 
  artificial 
  ageing. 
  The 
  results 
  

   given 
  in 
  the 
  successive 
  columns 
  of 
  the 
  table 
  are 
  for 
  measure- 
  

   ments 
  made 
  at 
  intervals 
  of 
  about 
  a 
  month. 
  They 
  are 
  for 
  

  

  