﻿162 
  On 
  the 
  Failure 
  of 
  German- 
  Silver 
  and 
  Platinoid 
  Wires. 
  

  

  wire 
  retain, 
  in 
  parts, 
  the 
  silver-grey 
  fracture. 
  As 
  a 
  rule, 
  

   however, 
  bad 
  specimens 
  have, 
  encroaching 
  on 
  the 
  silver-grey 
  

   area, 
  patches 
  of 
  purple, 
  yellow, 
  copper-colour, 
  and 
  brown, 
  

   with 
  frequent 
  cracks 
  and 
  fissures 
  that 
  communicate 
  more 
  or 
  

   less 
  with 
  the 
  surface 
  of 
  the 
  wire. 
  Very 
  bad 
  specimens 
  have, 
  

   at 
  times, 
  scarcely 
  any 
  of 
  the 
  silver-grey 
  colour 
  at 
  the 
  fracture. 
  

   The 
  patches 
  of 
  dark-purple, 
  yellow, 
  and 
  copper-colour 
  are 
  

   clearly 
  defined, 
  as 
  though 
  each 
  of 
  them 
  represented 
  a 
  distinct 
  

   alloy, 
  or 
  a 
  separate 
  mass 
  of 
  " 
  liquated 
  " 
  metal. 
  On 
  a 
  par- 
  

   ticular 
  bobbin, 
  brittleness 
  is 
  often 
  restricted 
  to 
  a 
  foot 
  or 
  two 
  

   of 
  the 
  wire. 
  Similar 
  remarks 
  apply 
  to 
  the 
  appearance 
  of 
  the 
  

   fractures 
  of 
  german-silver. 
  

  

  The 
  crevices 
  and 
  fissures 
  in 
  bad 
  specimens 
  are 
  easy 
  to 
  

   observe 
  with 
  a 
  microscope. 
  They 
  are 
  probably 
  developed 
  

   during 
  wire- 
  drawing, 
  as 
  the 
  result 
  of 
  pores 
  in 
  the 
  cast 
  

   metal. 
  Another 
  explanation 
  of 
  them 
  may 
  be 
  looked 
  for 
  in 
  

   the 
  " 
  liquation 
  " 
  of 
  some 
  of 
  the 
  metals 
  constituting 
  the 
  alloy. 
  

   By 
  this 
  action, 
  the 
  portion 
  of 
  the 
  alloy 
  that 
  first 
  solidifies 
  

   rejects 
  the 
  yet 
  molten 
  portions, 
  as 
  ice 
  rejects 
  foreign 
  matter. 
  

   As 
  cooling 
  continues, 
  the 
  various 
  constituents 
  become 
  isolated, 
  

   homogeneity 
  is 
  lost, 
  and 
  I 
  think 
  we 
  may 
  fairly 
  assume 
  that, 
  

   in 
  consequence, 
  the 
  strength 
  of 
  the 
  material 
  varies 
  from 
  point 
  

   to 
  point 
  of 
  its 
  mass, 
  so 
  that 
  in 
  passing 
  afterwards 
  through 
  the 
  

   die, 
  the 
  weaker 
  constituents 
  give 
  way, 
  and 
  the 
  general 
  

   structure 
  is 
  loosened. 
  Crystallization 
  and 
  internal 
  electrical 
  

   actions 
  may 
  also 
  result 
  in 
  local 
  weakness, 
  crevices, 
  and 
  

   fissures. 
  

  

  Keeping 
  in 
  mind 
  the 
  existence 
  of 
  these 
  fissures 
  it 
  is 
  a 
  

   simple 
  matter 
  to 
  account 
  for 
  the 
  fact 
  that 
  wires 
  deteriorate 
  

   more 
  quickly 
  under 
  a 
  small 
  stress 
  than 
  when 
  they 
  are 
  sub- 
  

   mitted 
  to 
  no 
  stress. 
  Apart 
  from 
  the 
  mechanical 
  weakening, 
  

   it 
  is 
  evident 
  that 
  any 
  extending 
  force 
  opens 
  the 
  crevices 
  and 
  

   makes 
  way 
  for 
  air 
  and 
  moisture— 
  it 
  may 
  also 
  facilitate 
  crys- 
  

   tallization. 
  And 
  clearly, 
  if 
  the 
  wires 
  are 
  being 
  used 
  for 
  

   electric 
  currents, 
  as 
  is 
  the 
  case 
  with 
  resistance-coils 
  on 
  arc- 
  

   light 
  circuits, 
  great 
  local 
  heating 
  occurs 
  at 
  these 
  weakened 
  

   sections, 
  and 
  fracture 
  is 
  inevitable. 
  This 
  was 
  probably 
  the 
  

   fate 
  of 
  the 
  voltmeter-wire 
  mentioned 
  in 
  example 
  (9) 
  above. 
  

   It 
  was 
  almost 
  certainly 
  the 
  fate 
  of 
  the 
  suspended 
  helices 
  (12). 
  

  

  From 
  what 
  precedes, 
  there 
  is 
  plenty 
  of 
  evidence 
  that 
  

   secondary 
  brittleness 
  is 
  fostered 
  by 
  tropical 
  heat 
  and 
  moisture. 
  

   A 
  good 
  wire, 
  free 
  from 
  incipient 
  cracks, 
  may 
  last 
  for 
  years, 
  

   or 
  may 
  be 
  proof 
  against 
  heat 
  and 
  moisture, 
  as 
  were 
  the 
  wires 
  

   in 
  example 
  (10). 
  But 
  if 
  there 
  are 
  any 
  fissures, 
  there 
  is 
  no 
  

   doubt 
  that 
  moisture 
  intrudes 
  upon 
  the 
  alloy 
  through 
  these 
  

   capillary 
  channels, 
  with 
  destructive 
  effect. 
  

  

  