﻿645 
  Mr. 
  J. 
  Patterson 
  on 
  the 
  Change 
  of 
  the 
  Electrical 
  

  

  Lenard 
  * 
  obtained 
  a 
  slight 
  change 
  of 
  resistance 
  for 
  german- 
  

   silver 
  in 
  a 
  magnetic 
  field. 
  He 
  does 
  not 
  state, 
  however, 
  the 
  

   composition 
  of 
  the 
  metal 
  he 
  used. 
  The 
  specimen 
  used 
  in 
  

   this 
  experiment 
  contained 
  about 
  1 
  2 
  per 
  cent, 
  of 
  nickel 
  ; 
  and 
  

   it 
  is 
  most 
  probable 
  that 
  the 
  different 
  results 
  are 
  due 
  to 
  

   different 
  compositions 
  of 
  the 
  alloy. 
  

  

  A 
  series 
  of 
  experiments 
  were 
  next 
  made 
  with 
  pure 
  copper 
  

   in 
  the 
  form 
  of 
  white-silk 
  covered 
  wire, 
  hard 
  drawn, 
  '002 
  inch 
  

   diameter. 
  The 
  two 
  spirals 
  had 
  resistances 
  of 
  24*04 
  and 
  22*92 
  

   ohms. 
  The 
  other 
  two, 
  of 
  german-silver, 
  had 
  24*12 
  and 
  22*95 
  

   ohms 
  respectively. 
  

  

  It 
  was 
  exceedingly 
  difficult 
  to 
  obtain 
  a 
  balance 
  on 
  the 
  

   bridge 
  for 
  any 
  length 
  of 
  time. 
  This 
  was 
  probably 
  due 
  to 
  

   heating 
  of 
  the 
  resistances, 
  for 
  as 
  they 
  were 
  insulated 
  with 
  

   paraffin, 
  the 
  heat 
  would 
  not 
  be 
  conducted 
  away 
  very 
  rapidly. 
  

   Besides, 
  one 
  of 
  the 
  spirals 
  being 
  between 
  two 
  large 
  pole-pieces 
  

   of 
  the 
  magnet, 
  the 
  heat 
  in 
  all 
  probability 
  was 
  conducted 
  away 
  

   from 
  that 
  spiral 
  at 
  a 
  different 
  rate 
  than 
  from 
  the 
  other. 
  The 
  

   galvanometer, 
  however, 
  always 
  indicated 
  an 
  increase 
  of 
  re- 
  

   sistance 
  when 
  the 
  magnetic 
  field 
  was 
  on, 
  but 
  when 
  the 
  field 
  

   was 
  put 
  off 
  and 
  the 
  bridge 
  again 
  tested 
  the 
  original 
  balance 
  

   had 
  altered 
  generally 
  in 
  the 
  direction 
  which 
  indicated 
  an 
  

   increase 
  of 
  resistance 
  in 
  the 
  compensating 
  spiral. 
  Any 
  

   change 
  in 
  the 
  resistance 
  of 
  the 
  coils 
  due 
  to 
  heating 
  would 
  

   therefore 
  tend 
  to 
  diminish 
  the 
  effect 
  due 
  to 
  the 
  magnetic 
  

   field 
  rather 
  than 
  increase 
  it. 
  This 
  shows 
  that 
  the 
  increase 
  

   in 
  resistance 
  of 
  the 
  test-coil 
  must 
  be 
  due 
  to 
  the 
  magnetic 
  

   field. 
  The 
  compensating 
  spiral 
  was 
  replaced 
  by 
  a 
  german- 
  

   silver 
  resistance 
  of 
  22*92 
  ohms. 
  This 
  gave 
  much 
  more 
  satis- 
  

   factory 
  results, 
  but 
  there 
  was 
  a 
  slow 
  change 
  from 
  time 
  to 
  time. 
  

  

  To 
  obtain 
  a 
  reading 
  the 
  bridge 
  was 
  first 
  balanced, 
  then 
  the 
  

   magnetic 
  field 
  put 
  on 
  and 
  the 
  bridge 
  again 
  balanced. 
  The 
  

   field 
  was 
  put 
  off, 
  and 
  the 
  bridge 
  tested 
  to 
  see 
  if 
  the 
  original 
  

   balance 
  had 
  altered. 
  If 
  it 
  had 
  changed 
  a 
  little, 
  the 
  experi- 
  

   ment 
  was 
  repeated 
  until 
  the 
  same 
  balance 
  was 
  obtained 
  after 
  

   as 
  before 
  the 
  field 
  was 
  put 
  on. 
  It 
  required 
  but 
  a 
  few 
  seconds 
  

   to 
  make 
  a 
  complete 
  observation. 
  

  

  If 
  A 
  is 
  the 
  resistance 
  of 
  the 
  test 
  spiral, 
  B, 
  C, 
  D 
  the 
  resist- 
  

   ances 
  in 
  the 
  other 
  arms 
  of 
  the 
  bridge, 
  being 
  in 
  multiple 
  

   arc 
  with 
  the 
  variable 
  resistance 
  R, 
  then 
  for 
  equilibrium 
  we 
  

   must 
  have 
  J) 
  A 
  

  

  C 
  + 
  R 
  

   A 
  

  

  a 
  

  

  * 
  Wied. 
  Ann. 
  vol. 
  xxxix. 
  p. 
  619 
  (1800). 
  

  

  